[WIP] partial port to bootstrap

Currently breaks everything (main page).
Partial port of header to bootstrap. Improvement of display on little screens.
Help required.
Well I really really don't find it nice at all which is why I started this PR. Firstly, it is completely not adapted to small screen. And as I am obliged to switch between mobile and desktop versions to make a quote on my cell phone, this PR will be handy. Secondly, I really (really) find ugly the fact that the button's hitbox doesn't have the same heigh than the navbar itself.