Don't fail when a LegacyDiffNote didn't store the right diff

Fixes https://sentry.gitlap.com/gitlab/gitlabcom/issues/8865/ and https://sentry.gitlap.com/gitlab/gitlabcom/issues/8754/

See merge request !5287

Revert "Merge branch 'gl-dropdown-issuable-form' into 'master'"

Temporarily fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19884

See merge request !5290

This reverts merge request !4970

Don't fail when Ci::Pipeline doesn't have a project

Fixes https://sentry.gitlap.com/gitlab/gitlabcom/issues/8870/

See merge request !5288

Display tooltip for mentioned users and groups

## What does this MR do?

Display tooltip for mentioned users and groups in Markdown.

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

I don't know.

## Why was this MR needed?

No tooltips were displayed.

## What are the relevant issue numbers?

fixes #17060 

## Screenshots

![user-tooltip](/uploads/501b7d1b2a5e6adcadefbc56b08422ec/user-tooltip.png)

![group-tooltip](/uploads/0152a452f9a1ed9fc86101b242f7e4c2/group-tooltip.png)


See merge request !5261

Upgrade to Rouge 2.0

## What does this MR do?

This MR is a refactor of the HTMLGitlab formatter. The original was copy-pasted from the legacy HTML formatter and adapted to suit our needs. In particular, this MR:

* Strips dead code and unused options
* Factors out the "wrapping" into the places that care about it - Bonzai and the view helpers
* Uses the Rouge 2.0 `token_lines` method to split tokens into lines, removing the custom code we used to do the same
* Uses the now-public `span` method instead of re-implementing it ourselves
* Removes options to not split into lines - the places where this feature wasn't being used are now using the plain `Formatters::HTML` instead of this class

See merge request !4691

Allow U2F devices to be used in Firefox

- Adds U2F support for Firefox
- Improve U2F feature detection logic
- Have authentication flow be closer to the spec (single challenge instead of a challenge for each `signRequest`)

- Closes #17341 
- Related to #15337 

See merge request !5177

Don't ask Heather to review documentation MR's



See merge request !5282

add project name and namespace to filename on project export

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19848

 Export filename now includes project and namespace path

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry 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 !5268

Fix mentioned users on diff notes

## Summary

`DiffNote`, and `LegacyDiffNote` returns empty array for `mentionable_attrs`, because `mentionable_attrs` is not inheritable by subclasses. The problem can be illustrated with this small sample:

```ruby
module Mentionable
  extend ActiveSupport::Concern

  module ClassMethods
    def attr_mentionable(attr)
      mentionable_attrs << [attr.to_s]
    end

    def mentionable_attrs
      @mentionable_attrs ||= []
    end
  end
end

class A
  include Mentionable

  attr_mentionable :foo
end

class B < A
end

A.mentionable_attrs
=> [["foo", {}]]

B.mentionable_attrs
=> []
```

Possible solution using `cattr_accessor`:

```ruby
module Mentionable
  extend ActiveSupport::Concern

  module ClassMethods
    def attr_mentionable(attr)
      mentionable_attrs << [attr.to_s]
    end
  end

  included do
    cattr_accessor :mentionable_attrs, instance_accessor: false do
      []
    end
  end
end

class A
  include Mentionable

  attr_mentionable :foo
end

class B < A
end

A.mentionable_attrs
=> [["foo"]]

B.mentionable_attrs
=> [["foo"]]

B.mentionable_attrs < [:bar]
=> [["foo"], ["bar"]]

A.mentionable_attrs
=> [["foo"], ["bar"]]
```

`mentionable_attrs` is inheritable by subclasses. If a subclass changes the value then that would also change the value for parent class. Similarly if parent class changes the value then that would change the value of subclasses too.

## What are the relevant issue numbers?

Fixes #19807 

Fixes #18022

/cc @stanhu  @DouweM @rspeicher 

See merge request !5243

Revert "Optimistic locking for Issue and Merge Requests"

The migration to add `lock_version` in !5146 to every issue and merge request takes too long on GitLab.com since it has to add a default value of 0 to every row. Unfortunately, Rails 4.2 doesn't work properly if the value is left as `nil`; anything using optimistic locking cannot be edited. 

This bug was fixed in Rails 5.0 via this commit: https://github.com/rails/rails/commit/13772bfa49325a8dc26410d2dcb555665a320f92. I suggest we revert this change for now, and when we upgrade to Rails 5.0 we can reintroduce this feature.

See merge request !5245

Update rubocop to 0.41.2

Adds a couple of cops to the TODO files, but I wanted [a specific cop](https://github.com/bbatsov/rubocop/pull/3135/files) which just had about 65 offences, so Ill do that later.

See merge request !4999

Disable PostgreSQL statement timeout during migrations

Long-running migrations may take more than the timeout allowed by the database. Disable the session's statement timeout to ensure migrations don't get killed prematurely.

See merge request !5263

Fix issues last update timestamp not preserved after importing a project

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19689 and https://gitlab.com/gitlab-org/gitlab-ce/issues/19792 (similar, but not the same)

Now `updated_at` should be the time from the original project.

See merge request !5253

Reduce padding on collapsed diff message

## What does this MR do?

Reduce the padding on the collapsed diff message so it takes up less space.

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

## Why was this MR needed?

Collapsed diffs took up too much space before.

## Screenshots (if relevant)

Before:

![image](/uploads/62c6fe19439bc3bc91bde83df72e7004/image.png)

After:

![image](/uploads/ec89230065c37fc4022d949d1233a90d/image.png)

## 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~~
  - [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 !5256

Changed collapsed assignee tooltip to users name

## What does this MR do?

- Changes the collapsed assignee tooltip the users name rather than the username
- Updates the tooltip after updating the assignee

## What are the relevant issue numbers?

Closes #19280, #19281

## Screenshots (if relevant)

![Screen_Shot_2016-06-28_at_17.55.52](/uploads/e6ccc1703f59966c978e731a841aabdc/Screen_Shot_2016-06-28_at_17.55.52.png)

See merge request !4976

navbar_icon was renamed to custom_icon

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4909 it's still using `navbar_icon` but now it's renamed to `custom_icon` in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5156

/cc @iamphill @smcgivern @annabeldunstone 

See merge request !5265

Address UI feedback for admin area

## What does this MR do?
Addresses @dzaporozhets's feedback for !4424

## Why was this MR needed?
!4424 was merged with some design issues which affected other areas of the app and broke consistency. This MR fixes that.

## What are the relevant issue numbers?
#3946 

## 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
  - [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)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5190

added changelog

Update permissons links to new page

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5210 updated the location in the docs, but we link to this page a bunch in the app too.

See merge request !5247

https://gitlab.com/gitlab-org/gitlab-ce/commit/c57471ddb456c9640f6d77128e1fc56c7a5b35b2

navbar_icon was used in:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4909