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

less/lib/forms.less

@@ -4,6 +4,7 @@
 .form-control {
   .box-shadow(none);
   border-width: 2px;
+  -webkit-appearance: none;
 
   &:focus,
   &.focus {