Migrate mention for MergeRequest notes

explain select * from notes, merge_requests where merge_requests.id = notes.noteable_id AND noteable_type = 'MergeRequest' and note like '%\@%'
Merge Join  (cost=34.51..48430159.16 rows=9663665 width=3168)
  Merge Cond: (notes.noteable_id = merge_requests.id)
  ->  Index Scan using index_notes_on_noteable_id_and_noteable_type on notes  (cost=0.57..42041324.54 rows=10084526 width=2455)
        Index Cond: ((noteable_type)::text = 'MergeRequest'::text)
        Filter: (note ~~ '%\@%'::text)

explain select * from notes, merge_requests where merge_requests.id = notes.noteable_id AND state_id =1 AND noteable_type = 'MergeRequest' and note like '%\@%'
Nested Loop  (cost=1.00..30561364.85 rows=474550 width=3168)
  ->  Index Scan using idx_merge_requests_on_target_project_id_and_iid_opened on merge_requests  (cost=0.43..1916559.80 rows=1658485 width=713)
  ->  Index Scan using index_notes_on_noteable_id_and_noteable_type on notes  (cost=0.57..17.25 rows=2 width=2455)
        Index Cond: ((noteable_id = merge_requests.id) AND ...