REFACTOR: Move Page title / focus / counts logic to service

We had a handful of methods attached to the root `Discourse` object
related to focus and notification counts.

This patch pulls them out into a service called `document-title` for
updating the title, and a component called `d-document` to attach
and listen for browser events related to focus.

It also removes some computed properties and observers in favor of plain
old Javascript objects.

test/javascripts/controllers/topic-test.js

@@ -10,7 +10,8 @@ moduleFor("controller:topic", "controller:topic", {
   needs: [
     "controller:composer",
     "controller:application",
-    "service:app-events"
+    "service:app-events",
+    "service:document-title"
   ],
   beforeEach() {
     this.registry.injection("controller", "appEvents", "service:app-events");