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.rb

@@ -1662,7 +1662,7 @@ class Post < ApplicationRecord
 
       if read_only
         begin
-          PostQueryBuilder.new(query).build(PostReadOnly.where("true"))
+          PostQueryBuilder.new(query, read_only: true).build
         rescue PG::ConnectionBad
           PostQueryBuilder.new(query).build
         end