Adjustments related to DiffNotes on diffs outside of a project's main repository

See merge request gitlab-org/gitlab-ce!29023

Automatically update MR merge-ref along merge status

Closes #58495

See merge request gitlab-org/gitlab-ce!28513

This turns Notable::RESOLVABLE_TYPES into a
`Notable.resolvable_types`. That allows us to override it in EE.

We have one auto merge strategy today - Merge When Pipeline
Succeeds.

In order to add more strategies for Merge Train feature,
we abstract the architecture to be more extensible.

Removed arguments

Fix spec

This introduces payload to the ServiceResponse with
the merge ref HEAD commit data

This couples the code that transitions the `MergeRequest#merge_status`
and refs/merge-requests/:iid/merge ref update.

In general, instead of directly telling `MergeToRefService` to update
the merge ref, we should rely on `MergeabilityCheckService` to keep
both the merge status and merge ref synced. Now, if the merge_status is
`can_be_merged` it means the merge-ref is also updated to the latest.

We've also updated the logic to be more systematic and less user-based.

This extends POST#:id/repository/commits to allow the optional parameter
`:start_project`, which will allow targeting other projects besides the
one derived from `:id`.

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/50850

We've already migrated all the legacy artifacts to the new realm,
which is ci_job_artifacts table.
It's time to remove the old code base that is no longer used.