Adding missing indexes migrations from taggings table

Closes #43927

See merge request gitlab-org/gitlab-ce!17563

Add index on section_name_id on ci_build_trace_sections table

Closes #44008

See merge request gitlab-org/gitlab-ce!17632

Clarify that the Pick Into X.Y label is added after approval

See merge request gitlab-org/gitlab-ce!17636

Remove double caching of Repository#empty?

Closes #43882

See merge request gitlab-org/gitlab-ce!17588

Fix link to Merge Request Coach

See merge request gitlab-org/gitlab-ce!17633

Resolve "Change Clear Runners cache UJS action to an API request"

Closes #43770

See merge request gitlab-org/gitlab-ce!17466

Add missing delete method in mattermost session

Closes #37367 and #40660

See merge request gitlab-org/gitlab-ce!16209

This was causing user deletions to time out.

Closes #44008

API requests were timing out because `tag_id` was missing an index. These
migrations were imported by running https://github.com/mbleigh/acts-as-taggable-on#post-installation:

```
bundle exec rake acts_as_taggable_on_engine:install:migrations
```

It looks like the `acts-as-tagglable` gem added the indexes in v4.0.0, but
when we upgraded from v3.5.0 (back in 2016 via
f571aeb5) we did not add them.

On staging, there are about 10.7 million rows on the `taggings` table. It took about 30 seconds for each index to be created.

On production, there are about 17.4 million rows, so I suspect the time to be about a minute per index.

Closes #43927

Backport changes from introducing GithubService interation in EE

See merge request gitlab-org/gitlab-ce!17607

Don't delete todos or unassign issues and MRs when a user leaves a project

Closes #43899

See merge request gitlab-org/gitlab-ce!17615

Port changes from EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4885 (Maintainer push to fork)

See merge request gitlab-org/gitlab-ce!17629

Removes confusing/unnecessary checkboxes when trying to configure an
integration. If there is only one supported event we don't need to
allow these to be individually disabled since the integration can be
disabled instead.

E.g. Project Integrations for GitHub, Bugzilla, Asana, Pipeline emails and Gemnasium

Allows integrations to override which triggers are configurable

backport of custom metrics for the common metrics section of CE

See merge request gitlab-org/gitlab-ce!17586

Update project activity when pushing to a wiki repository

See merge request gitlab-org/gitlab-ce!17621

Create CI/CD-only projects from GitHub -- CE backport

See merge request gitlab-org/gitlab-ce!17432

Fix accessibility typo

See merge request gitlab-org/gitlab-ce!17612

Backport relevant changes from EE https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4827 to CE

See merge request gitlab-org/gitlab-ce!17608

Move SidebarAssignees vue component

See merge request gitlab-org/gitlab-ce!17398

Resolve "Add a mention and link to doc for security implications in the Kubernetes cluster page"

Closes #43780 and #43781

See merge request gitlab-org/gitlab-ce!17486

# Conflicts:
#   locale/gitlab.pot

Add new frontend guide structure

See merge request gitlab-org/gitlab-ce!17611

Allow maintainers to push forks of a project for branches that have open MRs

Closes #22292

See merge request gitlab-org/gitlab-ce!17395

[ci skip] Merge branch 'master' into 43770-change-clear-runners-cache-ujs-action-to-an-axios-request

* master: (68 commits)
  Upgrade Workhorse to 4.0.0
  naming things
  Update GitLab Pages to v0.7.0
  Minor fixes in API doc
  Use Project#full_name instead of name_with_namespace
  Fix tests not completely disabling Gitaly
  Move OperationService#UserRemoveBranch
  Move OperationService#UserCreateBranch
  Move CommitService#Languages to OPT_OUT
  Move RefService#CreateBranch to OPT_OUT
  Move RefService#DeleteBranch to OPT_OUT
  Move OperationService#UserRevert to OPT_OUT
  Move OperationService#UserAddTag to OPT_OUT
  Move CommitService::CommitPatch to OPT_OUT
  Change to Pacific Time Zone
  Merge branch 'pages-6-1-gitlab-10-5' into 'security-10-5'
  Merge branch 'sh-fix-otp-backup-invalidation-10-5' into 'security-10-5'
  Remove wrong assumption about Runners cache GC
  Add CommonMark markdown engine
  add nginx_status monitoring details
  ...

Update GitLab Pages to v0.7.0

See merge request gitlab-org/gitlab-ce!17603

Resolve "Make margin around dropdown dividers 4px"

Closes #39444

See merge request gitlab-org/gitlab-ce!17517