add get_topic_url_by_external_id

Author: thomasrossetto

lib/discourse_api/api/topics.rb

@@ -119,6 +119,10 @@ def bookmark_topic(topic_id)
       def remove_topic_bookmark(topic_id)
         put("/t/#{topic_id}/remove_bookmarks.json")
       end
+
+      def get_topic_url_by_external_id(external_id)
+        get("/t/external_id/#{external_id}")
+      end
     end
   end
 end