Optimize metatag searches involving usernames, including user:,
approver:, appealer:, commenter:, upvoter:, etc.
Do `User.find_by_name` instead of `User.name_matches` because this
fetches the user upfront instead of doing it inside a subquery. Using a
subquery makes the SQL more complicated and leads to worse query plans.
This especially helps searches involving multiple username metatags.