Remove soft removals related code

Closes #37447

See merge request gitlab-org/gitlab-ce!15789

For each MR diff an extra 'SELECT COUNT()' is executed
to get number of commits for the diff. Overall time to get counts for
all MR diffs may be quite expensive. To speed up loading of MR info,
information about number of commits is stored in a MR diff's extra column.

Closes #38068

Backport authorized_keys

Closes gitlab-ee#3953

See merge request gitlab-org/gitlab-ce!16014

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

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