DEV: Make discourse-common an Ember addon. (#9578)

This is to help with the migration to Ember CLI. In the current running
version of Discourse everything should be the same as before, just with
a few extra files that are not used. However, using Ember CLI this can
be installed as an Ember addon.

Co-Authored-By: Jarek Radosz <jradosz@gmail.com>

app/assets/javascripts/discourse-common/ember-cli-build.js

@@ -0,0 +1,9 @@
+"use strict";
+
+const EmberAddon = require("ember-cli/lib/broccoli/ember-addon");
+
+module.exports = function(defaults) {
+  let app = new EmberAddon(defaults, {});
+
+  return app.toTree();
+};