Advance Search: Merge request with labels
Description
This is the first merge request to implement Advance Search with labels for merge requests. I will open another one for aggregations. These changes are under the search_mr_filter_label_ids
feature flag that we will roll out with #473796 (closed).
Previous related merge requests:
- Advanced Search: Index label_ids for merge requ... (!159377 - merged)
- Reindex merge requests to backfill label_ids (!159404 - merged)
Related to #464090 (closed).