REFACTOR: redo DiscourseTagging.filter_allowed_tags (#8328)

This method had grown into a monster. Its query had bugs
that I couldn't fix, and new features would be hard to add.
Also I don't understand how it all works anymore...
Replace it with common table expressions that can be queried
to generate the results we need, instead of subtracting
results using lots of "NOT IN" clauses.

Fixed are bugs with tag schemas that use combinations of
tag groups, parent tags, and one-tag-per-topic restrictions.
For example: https://meta.discourse.org/t/130991/6

app/controllers/tags_controller.rb

@@ -195,7 +195,8 @@ class TagsController < ::ApplicationController
   def search
     filter_params = {
       for_input: params[:filterForInput],
-      selected_tags: params[:selected_tags]
+      selected_tags: params[:selected_tags],
+      limit: params[:limit]
     }
 
     if params[:categoryId]
@@ -205,19 +206,14 @@ class TagsController < ::ApplicationController
     if params[:q]
       clean_name = DiscourseTagging.clean_tag(params[:q])
       filter_params[:term] = clean_name
-
-      # Prioritize exact matches when ordering
-      order_query = Tag.sanitize_sql_for_order(
+      filter_params[:order] = Tag.sanitize_sql_for_order(
         ["lower(name) = lower(?) DESC, topic_count DESC", clean_name]
       )
-
-      tag_query = Tag.order(order_query).limit(params[:limit])
     else
-      tag_query = Tag.limit(params[:limit])
+      filter_params[:order] = "topic_count DESC"
     end
 
     tags_with_counts = DiscourseTagging.filter_allowed_tags(
-      tag_query,
       guardian,
       filter_params
     )

app/controllers/topics_controller.rb

@@ -327,10 +327,9 @@ class TopicsController < ApplicationController
       if category && topic_tags = (params[:tags] || topic.tags.pluck(:name)).reject { |c| c.empty? }
         if topic_tags.present?
           allowed_tags = DiscourseTagging.filter_allowed_tags(
-            Tag.all,
             guardian,
             category: category
-          ).pluck("tags.name")
+          ).map(&:name)
 
           invalid_tags = topic_tags - allowed_tags