search: optimize username metatags.

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.

app/logical/post_query_builder.rb

@@ -266,7 +266,9 @@ class PostQueryBuilder
     when "none"
       Post.where(field => nil)
     else
-      Post.where(field => User.name_matches(username))
+      user = User.find_by_name(username)
+      return Post.none if user.nil?
+      Post.where(field => user)
     end
   end
 
@@ -278,7 +280,9 @@ class PostQueryBuilder
     elsif username == "none"
       Post.where("NOT EXISTS (#{subquery.to_sql})")
     elsif block.nil?
-      subquery = subquery.where(field => User.name_matches(username))
+      user = User.find_by_name(username)
+      return Post.none if user.nil?
+      subquery = subquery.where(field => user)
       Post.where("EXISTS (#{subquery.to_sql})")
     else
       subquery = subquery.merge(block.call(username))