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/utils/drawer.js

@@ -0,0 +1,13 @@
+export default class Drawer {
+  hide() {
+    $('body').removeClass('drawer-open');
+  }
+
+  show() {
+    $('body').addClass('drawer-open');
+  }
+
+  toggle() {
+    $('body').toggleClass('drawer-open');
+  }
+}