Ensure specs on sorting of issues in API are deterministic on MySQL

## What does this MR do?

Ensure a deterministic order of issues on the existing API issues specs

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

## Why was this MR needed?

To avoid random failures as described on #22057

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [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
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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)

## What are the relevant issue numbers?

Closes #22057

See merge request !6306

Fix two problematic bits of code that will be deprecated or broken in Rails 5.

Found in the Rails 5 MR: !5555

These are safe to use in Rails 4.2.7 as well as Rails 5.0.0, so I figured I'd backport them for the sake of making that merge request smaller.

The explanation for the mime_types.rb code is here: https://github.com/rails-api/active_model_serializers/issues/1027#issuecomment-126543577

See merge request !6214

Update templates.

See merge request !6163

Only add original author tag line when importing from GitHub if there isn't a linked GitLab account

## What does this MR do?
If there we've found a linked GitLab user for a creator of an issue or comment don't add the 'Created By:' line.

## What are the relevant issue numbers?

Closes #21569

See merge request !6081

Document how to track custom events

## What does this MR do?

This documents how one can track custom events.

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

Spelling, etc.

## Why was this MR needed?

This wasn't documented.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] 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)

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/22070

See merge request !6310

While using Gitlab UI to edit a file it changes the file permissions this MR aims to fix it

Closes #21092

See merge request !5979

Add information about user and manual build start to runner as variables

Closes #21825

See merge request !6201

Signed-off-by: Rémy Coutable <remy@rymai.me>

Make the cases clear instead of having guards

See merge request !6304

Fixes gitlab-org/gitlab-ce#22070

[ci skip]

Only check :can_resolve permission if the note is resolvable

See merge request !6298

MySQL could not have support for millisecond precision, depends on the MySQL version 
so we just create issues in different seconds in a deterministic way

Currently this permission check incurs a significant performance hit on loading issues
where it does not make sense to check this permission in the first place.

API: Use Search::GlobalService in projects search API

See merge request !6280

Fix API issues sorting

## What does this MR do?

Fix the sorting of issues in the API.

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

Instead of removing the '_at' suffix manually, we could add those versions to the `Sortable` concern instead.

## Why was this MR needed?

There were a couple of bugs:

* The global and project-specific issues endpoints wouldn't sort at all.
* Group sorting would work, but only if you applied two undocumented workarounds:
  * Always pass both `order_by` and `sort` (both are optional, so only one should be needed to change ordering).
  * Instead of passing `created_at` or `updated_at`, you needed to pass `created` or `updated`.

This makes the API implementation match the docs.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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)

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/983.

See merge request !6281

Also streamline the sorting part while we're at it.

That being done, there's currently a duplication between
`GET /projects/search/:query` and `GET /projects?search=:search`
so we might want to keep only the latter for 9.0...
Signed-off-by: Rémy Coutable <remy@rymai.me>

Add notification_settings API calls

## What does this MR do?
It adds API calls to get/update notification settings, globally and at the group/project level.

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
Notification settings were updated, but related API calls were never created.

## What are the relevant issue numbers?
gitlab-org/gitlab-ce#19359

## 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)

Closes #19359

See merge request !5632

Pull import sources from Gitlab::ImportSources

## What does this MR do?
`ApplicationSettings.create_from_defaults` defines an array of import sources, but we should be pulling them from `Gitlab::ImportSources.options` since that's what they're validated against.

## What are the relevant issue numbers?

Closes #21814

See merge request !6293

refactors update file

Improve PipelinesFinder spec so that it does not depend on hard-coded database IDs

Fixes failed builds such as https://gitlab.com/gitlab-org/gitlab-ce/builds/3919501

See merge request !6291

Fixes failed builds such as https://gitlab.com/gitlab-org/gitlab-ce/builds/3919501

Enable pipeline events by default

This should probably be included in !5620 but missed.

See merge request !6278