subject -> topic: Sweep "message.subject" in frontend.

These were the last remaining files.  After this, only
util.js has a non-email-related use of "subject".

static/js/message_store.js

@@ -135,7 +135,7 @@ exports.add_message_metadata = function (message) {
 
         topic_data.add_message({
             stream_id: message.stream_id,
-            topic_name: message.subject,
+            topic_name: util.get_message_topic(message),
             message_id: message.id,
         });