[WIP] partial port to bootstrap

Currently breaks everything (main page).
Partial port of header to bootstrap. Improvement of display on little screens.
Help required.
@Flaburgan We have the issues #4588 #5283 and #1960 for the header. I guess #1960 would need some bigger changes and Bootstrap already gives us a responsive header. That's why I planned to rewrite the header when porting everything to Bootstrap 3. I already tried to port some stuff to Bootstrap 3 and the header broke so at least when we port everything to Bootstrap 3 we would have to fix the header.

@AugierLe42e would like to port the header now and I don't see any problem here.