Add tags overlap field

This information is already available, so there's no extra processing.
To avoid conflicting with the current tags field, it was made into its
own field, and it uses a hash so that values can be quickly looked up.

app/logical/related_tag_query.rb

@@ -26,6 +26,14 @@ class RelatedTagQuery
     end
   end
 
+  def tags_overlap
+    if query =~ /\*/
+      {}
+    else
+      tags.map { |v| [v.name, v.overlap_count] }.to_h
+    end
+  end
+
   # Returns the top 20 most frequently added tags within the last 20 edits made by the user in the last hour.
   def recent_tags(since: 1.hour.ago, max_edits: 20, max_tags: 20)
     return [] unless user.present? && PostVersion.enabled?
@@ -78,6 +86,7 @@ class RelatedTagQuery
       query: query,
       category: category,
       tags: tags_with_categories(tags.map(&:name)),
+      tags_overlap: tags_overlap,
       wiki_page_tags: tags_with_categories(wiki_page_tags),
       other_wikis: other_wiki_pages.map { |wiki| [wiki.title, tags_with_categories(wiki.tags)] }.to_h
     }