When a user updates a merge request coming from a fork, they should
not be able to set `force_remove_source_branch` if they cannot push
code to the source project.

Otherwise developers of the target project could remove the source
branch of the source project by setting this flag through the API.

Because we don't have any destroy callbacks (or other logic
triggered on event destroy), there is no reason for deleting events
inefficiently one by one, instead we can use :delete_all.

These are the structural changes for supporting the EE feature of moving
"code_owner_approval_required" state from existing on a project to being
on the protected branches individually, allowing for CODEOWNER
validation on push events.

This commit adds pipeline.type key to PipelineEntity.
This key will be used in MR widget in the next iteration.

Prior to 12.1, rebase status was looked up directly from Gitaly. In
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14417 , a DB
column was added to track the status instead. However, we couldn't stop
looking at the gitaly status immediately, since some rebases may been
running across the upgrade.

Now that we're in 12.3, it is safe to remove the direct-to-gitaly
lookup. This also happens to fix a 500 error that is seen when viewing
an MR for a fork where the source project has been removed.

We still look at the Gitaly status in the service, just in case Gitaly
and Sidekiq get out of sync - I assume this is possible, and it's a
relatively cheap check.

Since we atomically check and set `merge_requests.rebase_jid`, we
should never enqueue two `RebaseWorker` jobs in parallel.

Splits auto-refreshing of MR widget into 2 requests:

- the one which uses etag-caching and invalidates the fields on change
- the one without caching

The idea is to gradually move all the fields to etag-cached endpoint

- Original EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14827