FEATURE: tag group options: limit usage of one tag per group, tags in a group can't be used unless a prerequisite tag is used

db/migrate/20160607213656_add_tag_group_options.rb

@@ -0,0 +1,6 @@
+class AddTagGroupOptions < ActiveRecord::Migration
+  def change
+    add_column :tag_groups, :parent_tag_id, :integer
+    add_column :tag_groups, :one_per_topic, :boolean, default: false
+  end
+end