Support multi-assignees on merge requests

This mainly handles the search and replace approach
to using assignees instead assignee for merge requests.

It reuses existing logic from Issuable, where now
issues _and_ merge requests handles multiple assignees.

The feature is behind a feature flag, though, there's no
way to switch back and forth to using the old and new tables.
That's because we'd need a "sync" approach: Once the feature
is off, we'd have to sync old table with new table, once it's
on, new table with old table. Using the new
`merge_request_assignees` altogether will make things simpler to
handle, even though it's a bit "riskier".