subs: Add stream_ui_updates.js module for managing ui elements.

This commit adds `stream_ui_updates.js` module. This module
will includes functions which will update different ui elements
(i.e. subscription button, subscriber count).

static/js/stream_ui_updates.js

@@ -0,0 +1,11 @@
+var stream_ui_updates = (function () {
+
+var exports = {};
+
+return exports;
+}());
+
+if (typeof module !== 'undefined') {
+    module.exports = stream_ui_updates;
+}
+window.stream_ui_updates = stream_ui_updates;