UX: Improve category filtering and include subcategories

* category_filtering
  1. report_top_referred_topics
  2. report_top_traffic_sources
  3. report_post_edit
* category_filtering with subcategory topics
  1. report_top_referred_topics
  2. report_top_traffic_sources
  3. report_post_edit
  4. report_posts
  5. report_topics
  6. report_topics_with_no_response
* category_filtering tests (without subcategory topics)
  1. report_posts
  2. report_topics_with_no_response
* subcategory topics tests `in_category_and_subcategories` in `topic_spec.rb`
  1. `in_category_and_subcategories` in `topic_spec.rb`
  2. topics, posts, flags and topics_with_no_response in `report_spec.rb`

app/models/post_action.rb

@@ -143,7 +143,7 @@ class PostAction < ActiveRecord::Base
     result = unscoped.where(post_action_type_id: post_action_type)
     result = result.where('post_actions.created_at >= ?', opts[:start_date] || (opts[:since_days_ago] || 30).days.ago)
     result = result.where('post_actions.created_at <= ?', opts[:end_date]) if opts[:end_date]
-    result = result.joins(post: :topic).where('topics.category_id = ?', opts[:category_id]) if opts[:category_id]
+    result = result.joins(post: :topic).merge(Topic.in_category_and_categories(opts[:category_id])) if opts[:category_id]
     result.group('date(post_actions.created_at)')
       .order('date(post_actions.created_at)')
       .count