Originally from branch 'fix-authorized-keys-enabled-default-2738' via merge request https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2240

Removed background migrations which were intended to fix state after using Gitlab
without a default having been set

Squashed commits:
Locally, if Spring was not restarted, `current_application_settings` was still cached, which prevented the migration from editing the file. This will also ensure that any app server somehow hitting old cache data will properly default this setting regardless.
Retroactively fix migration
  This allows us to identify customers who ran the broken migration. Their `authorized_keys_enabled` column does not have a default at this point.
  We will fix the column after we fix the `authorized_keys` file.
Fix authorized_keys file if needed
Add default to authorized_keys_enabled setting
  Reminder: The original migration was fixed retroactively a few commits ago, so people who did not ever run GitLab 9.3.0 already have a column that defaults to true and disallows nulls. I have tested on PostgreSQL and MySQL that it is safe to run this migration regardless.
  Affected customers who did run 9.3.0 are the ones who need this migration to fix the authorized_keys_enabled column.
  The reason for the retroactive fix plus this migration is that it allows us to run a migration in between to fix the authorized_keys file only for those who ran 9.3.0.
Tweaks to address feedback
Extract work into background migration
Move batch-add-logic to background migration
  Do the work synchronously to avoid multiple workers attempting to add batches of keys at the same time.
  Also, make the delete portion wait until after adding is done.
Do read and delete work in background migration
Fix Rubocop offenses
Add changelog entry
Inform the user of actions taken or not taken
Prevent unnecessary `select`s and `remove_key`s
Add logs for action taken
Fix optimization
Reuse `Gitlab::ShellAdapter`
Guarantee the earliest key
Fix migration spec for MySQL

See merge request gitlab-org/gitlab-ee!3323

Backported as part of authorized_keys in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16014

Backport to CE, originally branch 'evn-ssh-clarify-docs

See merge request gitlab-org/gitlab-ee!3753

Originally branch 'mk-toggle-writing-to-auth-keys-1631'

See merge request !2004

Squashed commits:
Add authorized_keys_enabled to Application Settings
Ensure default settings are exposed in UI
Without this change, `authorized_keys_enabled` is unchecked when it is nil, even if it should be checked by default.
Add “Speed up SSH operations” documentation
Clarify the reasons for disabling writes
Add "How to go back" section
Tweak copy
Update Application Setting screenshot

Add find key by base64 key or fingerprint to the internal API

See merge request !250

Squashed changes:
Add unique index to fingerprint
Add new index to schema
Add internal api to get ssh key by fingerprint
Change API endpoint to authorized_keys
Add InsecureKeyFingerprint that calculates the fingerprint without shelling out
Add require for gitlab key fingerprint
Remove uniqueness of fingerprint index
Remove unique option from migration
Fix spec style in fingerprint test
Fix rubocop complain
Extract insecure key fingerprint to separate file
Change migration to support building index concurrently
Remove those hideous tabs

Fix custom name in branch creation for issue in Firefox

Closes #41563

See merge request gitlab-org/gitlab-ce!16244

API: get participants from merge_requests & issues

Closes #40986

See merge request gitlab-org/gitlab-ce!16187

Resolve "Create secrets failed for Auto DevOps with latest stable of kubectl"

Closes #41376

See merge request gitlab-org/gitlab-ce!16218

Backport 'Rebase' feature from EE to CE

Closes #40301

See merge request gitlab-org/gitlab-ce!16071

Remove the Project#repo method

See merge request gitlab-org/gitlab-ce!16238

Add docs about end-to-end testing / GitLab QA tests

Closes #39637

See merge request gitlab-org/gitlab-ce!16043

Expose project_id on /api/v4/pages/domains

See merge request gitlab-org/gitlab-ce!16200

Don't use Gitlab::Shell#import_repository on local paths

See merge request gitlab-org/gitlab-ce!16228

[EE-backport] Add group boards API endpoint

See merge request gitlab-org/gitlab-ce!16194

Backport of methods and components added in EE

See merge request gitlab-org/gitlab-ce!16208

Update redis-rack to 2.0.4

See merge request gitlab-org/gitlab-ce!16251

Update settings.md

See merge request gitlab-org/gitlab-ce!16233

Merge branch 'master' of gitlab.com:gitlab-org/gitlab-ce into 41563-fix-branch-creation-from-issue-in-firefox

Fix import project url not updating project name

Closes #40607

See merge request gitlab-org/gitlab-ce!16120

Add id to modal.vue to support data-toggle="modal"

Closes #41584

See merge request gitlab-org/gitlab-ce!16189

Up until version 2.0.3 redis-rack included a "rake" binary that would
overwrite/hijack the one provided by Rake itself. Unfortunately the
binary provided by redis-rack would produce errors in many cases. See
https://github.com/redis-store/redis-rack/pull/34 for more info.

Refactor cluster show page

Closes #41335

See merge request gitlab-org/gitlab-ce!16089

Allow local tests to use a modified Gitaly

Closes gitaly#833

See merge request gitlab-org/gitlab-ce!16220

Resolve "Verify integrity of repositories"

Closes #40228

See merge request gitlab-org/gitlab-ce!15931

Add online attribute to runner api entity

Closes #25774

See merge request gitlab-org/gitlab-ce!11750

Reroute batch blobs to single blob RPC

See merge request gitlab-org/gitlab-ce!16082

Prepare Gitlab::Git::Repository#rebase for Gitaly migration

See merge request gitlab-org/gitlab-ce!16192

Enclose props in double quotes for Item Stats in Groups tree app

See merge request gitlab-org/gitlab-ce!16231

Update some Gitaly annotations in Gitlab::Shell

See merge request gitlab-org/gitlab-ce!16226