PostSetPresenters::Post#related_posts: clean up metatag parsing (#2894).

* Fix `#related_tags` to use `Tag.has_metatag?`.
* Fix Tag::SUBQUERY_METATAGS and Tag::METATAGS to be arrays instead of regexes.

test/unit/tag_test.rb

@@ -242,8 +242,8 @@ class TagTest < ActiveSupport::TestCase
       should_not allow_value("café").for(:name).on(:create)
       should_not allow_value("東方").for(:name).on(:create)
 
-      metatags = Tag::METATAGS.split("|") + Tag::SUBQUERY_METATAGS.split("|") + TagCategory.mapping.keys
-      metatags.split("|").each do |metatag|
+      metatags = Tag::METATAGS + TagCategory.mapping.keys
+      metatags.each do |metatag|
         should_not allow_value("#{metatag}:foo").for(:name).on(:create)
       end
     end