Deprecates global use of Discourse.MessageBus

We can use DI for this, which makes it easier for plugins to subscribe
to the message bus.

test/javascripts/test_helper.js

@@ -62,7 +62,7 @@ sinon.config = {
 window.assetPath = function() { return null; };
 
 // Stop the message bus so we don't get ajax calls
-Discourse.MessageBus.stop();
+window.MessageBus.stop();
 
 // Trick JSHint into allow document.write
 var d = document;