Add ticket-based Kerberos sign-ins

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/745

This is meant as a replacement for omniauth-kerberos. This MR does not rip out or disable omniauth-kerberos, however, because that has the risk of locking out all Kerberos users on Kerberos-enabled GitLab server that does not have a service keytab yet.

On GitLab installations that have a keytab, namely those which have 'git access via SPNEGO' enabled, this change will automatically enable ticket-based sign-ins. 

See merge request !548

Fix autoloading issue in test environment

Always prefer to use the full class namespace when specifying a superclass inside a module, because autoloading can occur in a different order between execution environments.

```ruby
module Foo
  class Base
    def initialize
      raise "This is the wrong super class!"
    end
  end
end

module Foo
  module Bar
    class Sample < Base
      # Inherits from ::Foo::Base because Foo::Bar::Base isn't defined yet
    end
  end
end

module Foo
  module Bar
    class Base
      def initialize
        puts "Success!"
      end
    end
  end
end

> Foo::Bar::Sample.new
=> RuntimeError: This is the wrong super class!
```

The trouble occurs when Rails has already autoloaded `Gitlab::SearchResults` and not `Gitlab::Elastic::SearchResults`. In this case, Rails will never need to autoload `Gitlab::Elastic::SearchResults` because `Gitlab::SearchResults` already matches the superclass. This is why this [build](https://gitlab.com/gitlab-org/gitlab-ee/builds/2464466) only fails when the spec run after `spec/lib/gitlab/search_results_spec.rb`.

/cc @DouweM @rspeicher @vsizov 



See merge request !571

Always prefer to use the full class namespace when specifying a
superclass inside a module, because autoloading can occur in a
different order between execution environments.

Uninitialized constant ElasticIndexerWorker::Client

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/772

See merge request !554

CE upstream



See merge request !570

CE upstream

@timothyandrew Please resolve:

```
spec/lib/gitlab/git_access_spec.rb
lib/gitlab/checks/change_access.rb
lib/gitlab/git_access.rb
lib/gitlab/git_access_wiki.rb
```

See merge request !568

Use value of `yaml_variables` and `when` from config_processor if undefined

## What does this MR do?
Uses `yaml_variables` and `when` from config_processor if values for these are undefined.

## Why was this MR needed?
Old builds doesn't have this columns initialised. 
Thus makes the retries on these builds to not work properly.
This is regression introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5296.

## Does this MR meet the acceptance criteria?

- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)


See merge request !5342

Added redirect_to_referer to login link on issues

## What does this MR do?

Adds a `redirect_to_referer` param to login & register links on issues when not logged in. This makes the login box redirect back to the issue after login.

## What are the relevant issue numbers?

Closes #19968 

See merge request !5344

Add support for manual CI actions

## What does this MR do?
This implements a `when: manual` which allows a jobs to be marked as manual actions.
Manual actions have to be explicitly executed by developers.

## What are the relevant issue numbers?
This is to solve: https://gitlab.com/gitlab-org/gitlab-ce/issues/17010

See merge request !5297

Simplify entities for branches and tags API

## What does this MR do?

As talked in !5208, I improved the entity for branches and tags.

## Does this MR meet the acceptance criteria?

~~- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added~~
~~- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
~~- [ ] API support added~~
~~- Tests~~
  ~~- [ ] Added for this feature/bug~~
  ~~- [ ] All builds are passing~~
~~- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)~~
~~- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)~~
~~- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)~~

See merge request !5343

Resolve "Add icons for branch and tag in pipeline list"

## What does this MR do?
Adds branch or tag icon on pipeline ref

## What are the relevant issue numbers?
Closes #19751    
Part of #18920 

## Screenshots (if relevant)
![Screen_Shot_2016-07-13_at_1.42.54_PM](/uploads/d90706a76cf87a6cc8c25dca9a71affa/Screen_Shot_2016-07-13_at_1.42.54_PM.png)

With icons closer to tag/branch name/sha:   
![Screen_Shot_2016-07-15_at_11.23.48_AM](/uploads/411338f811bafbd314dbab5553fa46d4/Screen_Shot_2016-07-15_at_11.23.48_AM.png)

See merge request !5239

Added Rake task for tracking deployments

## What does this MR do?

This MR adds a Rake task to track the current version of GitLab in InfluxDB.

## Are there points in the code the reviewer needs to double check?

We may want to document this Rake task in some Markdown file, but I'm not sure what the best place for this would be.

## Why was this MR needed?

We want to have a more accurate overview of when deployments occurred. 

## What are the relevant issue numbers?

https://gitlab.com/gitlab-com/infrastructure/issues/98

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] ~~API support added~~
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5320

Closes #19968

Api delete todos

## What does this MR do?

It changes the `DELETE /todos` to only return the number of marked todos.

## Why was this MR needed?

Before it returned the marked todos as an paginated array. Since we deleted/marked the todos, there was no way to get the rest of the todos.

## What are the relevant issue numbers?

Closes #19678

## Does this MR meet the acceptance criteria?

~~- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added~~
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5341

This simply inserts the current GitLab version in the "deployments"
measurement.

Fixes gitlab-com/infrastructure#98

Change color of today issue highlight



See merge request !5329

API: Expose 'developers_can_push' for branches

## What does this MR do?

Adds support for the `developers_can_push` flag for the branches API. It also supports creating protected branches with that flag.

## Are there points in the code the reviewer needs to double check?

The API call requires an optional boolean parameter `developers_can_push`. If it is not either `true` or `false`, it should not update the value. I'm not sure if this is the right way to do it.

Right now it always returns `200`. Maybe the `PUT` method should return different status codes depending if it really created or updated a branch.

## What are the relevant issue numbers?

Closes #12735 

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5208

Fix MR diff encoding issues exporting projects

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19338

See merge request !5039

WIP - trying to replicate UTF-8 error

fix spec

fixing encoding issue and another spec, to do with MR diffs

fix issue and spec failure

Add changelog and bumped up I/E version

fix spec based on feedback - omitted target project

Small ui improvements

* make file tree row 2px smaller for more compact view
* make sub navigation  background a bit darker for better contrast with main navigation

cc @hazelyang @alfredo1 

See merge request !5315

Update documentation for build artifact dependencies

## What does this MR do?

Update documentation for build artifact dependencies

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

#19892 

## What are the relevant issue numbers?

#19892 

## Screenshots (if relevant)


See merge request !5332

Update MAINTENANCE.md with our latest policy

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19958

See merge request !5339

[ci skip]