(Also removes duplicate changelog entry.)

This reverts merge request !6429

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

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

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

Closes #22354

- Fixed and added specs.
- Added different namespace options depending on user privilages
- Updated docs.

* Created a force=true param that will continue with the previous
  behaviour of the unsubscribe method
* Created a filter for not-logged users so they see a unsubsribe
  confirmation page
* Added the List-Unsubscribe header on emails so the email client can
  display it on top

Also incorporate feedback

The lock in turn is only obtained when actually needed, reducing some
load on Redis.

Fixes gitlab-org/gitlab-ce#22213

Revert "Revert all changes introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6043"

This reverts commit 6d43c95b.

* Remove call to Milestone#save! instead just Milestone#save
* Add safety specs to stop a regression

Fixes #22033

Better import of labels, milestones and protected branches. Updated relevant specs.
Loose pipeline validation on importing, so it does not fail when there are missing fields, which are not validated at DB level. Also, updated spec with relevant test.

A customer ran into an issue where a Sidekiq task retried over and over, leading to duplicate
master branches in their protected branch list.

Closes #22177

It's mainly intended to improve Elasticsearch indexing performance
(through multi_json gem), but other gems could benefit from it too, like
grape.

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

Closes #22260

[ci skip]
Signed-off-by: NRémy Coutable <remy@rymai.me>