Fix #4035: The Related Tag JSON endpoint is returning the wrong information

* Fix /related_tags.json to return a list of tags in the `other_wikis` field.

* Add support for /related_tags.xml.

app/controllers/related_tags_controller.rb

@@ -4,11 +4,7 @@ class RelatedTagsController < ApplicationController
 
   def show
     @query = RelatedTagQuery.new(query: params[:query], category: params[:category], user: CurrentUser.user)
-    respond_with(@query) do |format|
-      format.json do
-        render :json => @query.to_json
-      end
-    end
+    respond_with(@query)
   end
 
   def update