search: move misc search parsing helpers to PostQueryBuilder.

* Move various search parser helper methods (`has_metatag?`,
  `is_single_tag?` et al) from PostSets and the Tag model to
  PostQueryBuilder.

* Fix various minor bugs stemming from trying to check if a search query
  contains certain metatags using regexes or other adhoc techniques.

app/presenters/post_presenter.rb

@@ -163,7 +163,7 @@ class PostPresenter
   end
 
   def has_sequential_navigation?(params)
-    return false if Tag.has_metatag?(params[:q], :order, :ordfav, :ordpool)
+    return false if PostQueryBuilder.new(params[:q]).has_metatag?(:order, :ordfav, :ordpool)
     return false if params[:pool_id].present? || params[:favgroup_id].present?
     return CurrentUser.user.enable_sequential_post_navigation
   end