Remove code related to object hierarchy and MySQL

Closes #65056 and #65055

See merge request gitlab-org/gitlab-ce!31095

Support title and desc on merge w/ push option

See merge request gitlab-org/gitlab-ce!31068

These are not required because MySQL is not
supported anymore

* Limits raw requests to 300 per minute and per raw path.
* Add a new attribute to ApplicationSettings so user can change this
value on their instance.
* Uses Gitlab::ActionRateLimiter to limit the raw requests.
* Add a new method into ActionRateLimiter to log the event into auth.log

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/48717

MergeRequests::PushOptionsHandlerService has been updated to allow
creating and updating merge requests with the `title` and
`description` set using git push options.

To create a new merge request and set its title and description:

      git push -u origin -o merge_request.create \
        -o merge_request.title="My title" \
        -o merge_request.description="My description"

To update an existing merge request and set its title and
description:

      git push -u origin -o merge_request.title="My title" \
        -o merge_request.description="My description"

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/64320

Although `flat_map` is equivalent to `map` + `flatten(1)`
(note the level 1) we can apply this same refactoring to all cases.

Add bulk update sidebar to merge requests template
Add js to toggle sidebar
Fix bulk update button in issues template
Add specs for merge requests in bulk update service

Extra logging for new live trace architecture

See merge request gitlab-org/gitlab-ce!30892

Make pipeline emails respect group email setting

Closes #63485

See merge request gitlab-org/gitlab-ce!30907