post_tags_match: replace joins with subqueries.

Refactor various post_tag_match methods to use subqueries instead of joins.

This simplifies things inside PostQueryBuilder, since now we can assume
we're always dealing with a Post relation, rather than some other table
joined with the posts table.

app/models/note.rb

@@ -18,7 +18,7 @@ class Note < ApplicationRecord
     end
 
     def post_tags_match(query)
-      PostQueryBuilder.new(query).build(self.joins(:post)).reorder("")
+      where(post_id: PostQueryBuilder.new(query).build.reorder(""))
     end
 
     def for_creator(user_id)