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/post_replacement.rb

@@ -21,7 +21,7 @@ class PostReplacement < ApplicationRecord
   concerning :Search do
     class_methods do
       def post_tags_match(query)
-        PostQueryBuilder.new(query).build(self.joins(:post))
+        where(post_id: PostQueryBuilder.new(query).build.reorder(""))
       end
 
       def search(params = {})