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/search.less

@@ -1,6 +1,8 @@
-.search-box {
-  & input:focus, &.active input, & .search-results {
-    width: 400px;
+@media @tablet, @desktop, @desktop-hd {
+  .search-box {
+    & input:focus, &.active input, & .search-results {
+      width: 400px;
+    }
   }
 }
 .search-results {