approvals.merge_request_id is not indexed

This column is used for filtering, but is not indexed. For example:

SELECT "users".* 
FROM "users" 
WHERE "users"."id" = 1245 
AND ("users"."id" NOT IN (SELECT "approvals"."user_id" FROM "approvals" WHERE "approvals"."merge_request_id" = 1234))  
ORDER BY "users"."id" DESC;

We should add an index to this column.