Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4641#note_12444891

* prefer-assign_to:
  Give 409 Conflict whenever the runner was already enabled
  We're checking return value rather than rescuing exceptions
  Prefer Runner#assign_to instead of creating directly

* master: (147 commits)
  Update CHANGELOG
  Remove deprecated issues_tracker and issues_tracker_id from project
  Schema doesn’t reflect the changes of the last 3 migrations
  Revert CHANGELOG
  Also rename "find" in the specs
  Change to new Notes styleguide
  Add guide on changing a document's location
  Change logs.md location in README
  Move logs/logs.md to administration/logs.md
  Make "four phase test"
  Only show branches for revert / cherry-pick
  Instrument all Banzai::ReferenceParser classes
  Removed old comment from update_column_in_batches
  Update columns in batches until no rows are left
  Remove counters from Pipeline navigation
  Handle NULL migration errors in migration helpers
  Fix typo causing related branches to Error 500
  Improved SVG sanitizer specs to include smoke tests for clean.
  Refactored SVG sanitizer
  Added SVG sanitizer fix to the changelog
  ...

Feedback from:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4093#note_12413950

Retry spinach tests in case of failure using rerun reporter

## What does this MR do?

Fixes Spinach tests to retry on tests on master


See merge request !4539

Remove deprecated issues_tracker and issues_tracker_id from project model

Closes #3941

See merge request !4603

Update schema.rb for most recent migrations

Fixes #18532

See merge request !4636

The schema doesn’t reflect the changes of the last 3 migrations:

* 20160610140403_remove_notification_setting_not_null_constraints.rb
* 20160610201627_migrate_users_notification_level.rb
* 20160610301627_remove_notification_level_from_users.rb

Remove counters from Pipeline navigation

* counters are heavy from UI perspective
* duplicates information from tabs below
* shows 0 for less active projects. Jumps from 0 to N constantly. 

I think we should render counters only when necessary and only for filters ( tabs on white bg ).  Exception:  issues, merge requests in top navigation as status of project popularity and action required from maintainers 


cc @grzesiek @ayufan @jschatz1 

See merge request !4617

Improve Gitlab::Auth method names

Auth.find was a very generic name for a very specific method.
Auth.find_in_gitlab_or_ldap was inaccurate in GitLab EE where it also
looks in Kerberos.


See merge request !4589

Enable Style/MultilineBlockChain rubocop style cop

Avoid multi-line chains of blocks.

See #17478.

See merge request !4349

Update brakeman from 3.2.1 to 3.3.2

Removes a few dependencies.

Changelog: https://github.com/presidentbeef/brakeman/blob/master/CHANGES

See merge request !4627

Fix typo causing related branches to Error 500

From Sentry: https://sentry.gitlap.com/gitlab/gitlabcom/issues/6154/

See merge request !4611

Fixes bulk-assign label for multiple issues not having the same labels

## What does this MR do?
Fixes a bug when bulk-assigning a label to multiple issues while the label is present in on the issues on the selection.

## Screenshots (if relevant)

**Before Bugfix**

<img src="/uploads/ad1f290bcf3930177a3a71c69cbe5325/before-bugfix.gif" width="700"/>

**After Bugfix**

<img src="/uploads/1f04d6bf027806fb13ca3773febda744/bugfix.gif" width="700"/>

## Does this MR meet the acceptance criteria?

- [x] Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)

See merge request !4602

Added when to artifacts

The syntax for this will be:

```
job:
  artifacts:
    when: on_failure
```

cc @grzesiek @tmaczukin 

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/4034


See merge request !4201

Cache only apt and ruby from vendor

## What does this MR do?

Optimises caching mechanism when testing gitlab-ce/ee repositories.


See merge request !4619

Instrument all Banzai::ReferenceParser classes

## What does this MR do?

This MR instruments all the classes in the `Banzai::ReferenceParser` namespace.

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

No.

## Why was this MR needed?

The classes in question weren't instrumented meaning we have no idea how they're performing in production.

## What are the relevant issue numbers?

None.

## 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~~
- [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 !4620

Add guide on changing a document's location

## What does this MR do?

Add a documentation styleguide when changing a document's location.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/3349


See merge request !4624

Change to new Notes styleguide



See merge request !4625

[ci skip]

[ci skip]

Fix migration helper race conditions

## What does this MR do?

This MR fixes two problems with the migration helpers:

1. An error in `change_column_null` would not drop the previously created column
2. `update_column_in_batches` would rely on the number of rows in a table to determine how many to update. This meant that newly inserted rows (after the `COUNT`) would not be taken into account.

This MR also removes an outdated comment for `update_column_in_batches`.

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

No.

## Why was this MR needed?

See above.

## What are the relevant issue numbers?

Fixes #18483

## Does this MR meet the acceptance criteria?

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

See merge request !4618

Only show branches for revert / cherry-pick

## What does this MR do?

Stop showing tags in the revert and cherry-pick select options. You can't change a tag anyway.

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

Uncertain.

## Why was this MR needed?

Showing tags doesn't make any sense and will just throw an exception if the user tries to cherry-pick or revert onto a tag.

## What are the relevant issue numbers?

Fixes #18377.

## Screenshots (if relevant)

On the GitLab CE repo:

![image](/uploads/9c5a6f09300a7c46d0a794bef2956992/image.png)

## 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
  - [x] Added for this feature/bug
  - [ ] 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 !4596

Change logs.md location in README

## What does this MR do?

Forgotten link from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4622

See merge request !4623

Move logs/logs.md to administration/logs.md

## What does this MR do?

Move `logs.md` documentation under `administration/` where it fits better.

## Why was this MR needed?

For a better doc structure.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

See merge request !4622

[ci skip]

Remove notification level from user model

part of #3359 

See merge request !4494

[ci skip]