[ticket/12662] Reorganize and modernize the header navbar

https://tracker.phpbb.com/browse/PHPBB3-12662
http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=45679&start=60

__TODO:__
- [x] Fix RTL notifications dropdown positioning in (very small) responsive mode
- [x] Fix IE8 notifications body overflow in compact mode
- [x] Fix username dropdown arrow when user doesn't have `<span>` around his name (when user has no color)
- [x] Fix username dropdown arrow position and dropdown container position when `navbar-header-username-append` event is used

![updated](https://cloud.githubusercontent.com/assets/3015760/3210923/13dcb72a-eef0-11e3-85a3-b553fad8fbda.png)

Logged out
![loggedout](https://cloud.githubusercontent.com/assets/3015760/3208288/085885d2-ee28-11e3-89f8-f733f0dbb24e.png)

PHPBB3-12662