Added a timeline for Direct statuses

* Lists all Direct statuses you've sent and received
* Displayed in Getting Started
* Streaming server support for direct TL

app/javascript/mastodon/features/ui/util/async-components.js

@@ -26,6 +26,10 @@ export function HashtagTimeline () {
   return import(/* webpackChunkName: "features/hashtag_timeline" */'../../hashtag_timeline');
 }
 
+export function DirectTimeline() {
+  return import(/* webpackChunkName: "features/direct_timeline" */'../../direct_timeline');
+}
+
 export function Status () {
   return import(/* webpackChunkName: "features/status" */'../../status');
 }