Significantly improve mobile UX

Most of #137 done.

- Use FastClick to make everything feel more responsive
- Use transforms for animations to make them silky smooth
- Style the drawer the same as the header to keep things simple
- Revert to fixed composer, but allow it to be minimised
- Add a separate notifications page for mobile so it’s easy to go back
- Add indicator to the menu button when there are unread notifications
- Close the drawer when navigating away
- Make dropdowns/modals scrollable
- Many other mobile tweaks and bug fixes

Didn’t take much care to keep CSS clean, due to #103

js/forum/src/initializers/routes.js

@@ -2,6 +2,7 @@ import IndexPage from 'flarum/components/index-page';
 import DiscussionPage from 'flarum/components/discussion-page';
 import ActivityPage from 'flarum/components/activity-page';
 import SettingsPage from 'flarum/components/settings-page';
+import NotificationsPage from 'flarum/components/notifications-page';
 
 export default function(app) {
   app.routes = {
@@ -16,7 +17,8 @@ export default function(app) {
     'user.discussions': ['/u/:username/discussions', ActivityPage.component({filter: 'startedDiscussion'})],
     'user.posts':       ['/u/:username/posts', ActivityPage.component({filter: 'posted'})],
 
-    'settings':         ['/settings', SettingsPage.component()]
+    'settings':         ['/settings', SettingsPage.component()],
+    'notifications':    ['/notifications', NotificationsPage.component()]
   };
 
   app.route.discussion = function(discussion, near) {