Convert a lot of Globals to ES6 modules

test/javascripts/models/session-test.js.es6

@@ -1,6 +1,8 @@
+import Session from "discourse/models/session";
+
 module("Discourse.Session");
 
 test('highestSeenByTopic', function() {
-  var session = Discourse.Session.current();
+  var session = Session.current();
   deepEqual(session.get('highestSeenByTopic'), {}, "by default it returns an empty object");
 });