FEATURE: Tag synonyms

This feature adds the ability to define synonyms for tags, and the ability to merge one tag into another while keeping it as a synonym. For example, tags named "js" and "java-script" can be synonyms of "javascript". When searching and creating topics using synonyms, they will be mapped to the base tag.

Along with this change is a new UI found on each tag's page (for example, `/tags/javascript`) where more information about the tag can be shown. It will list the synonyms, which categories it's restricted to (if any), and which tag groups it belongs to (if tag group names are public on the `/tags` page by enabling the "tags listed by group" setting). Staff users will be able to manage tags in this UI, merge tags, and add/remove synonyms.

db/migrate/20191113193141_add_target_tag_id_to_tags.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddTargetTagIdToTags < ActiveRecord::Migration[6.0]
+  def change
+    add_column :tags, :target_tag_id, :integer
+  end
+end