[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
I suggest we don't use template vars for this, but simply let jQ figure out if there's anything to display at all. That way we can catch any possible scenario.

Will fix it tomorrow.