[WIP] partial port to bootstrap

Currently breaks everything (main page).
Partial port of header to bootstrap. Improvement of display on little screens.
Help required.
@svbergerem : In particular, I have a problem with that part. I don't really understand how the icons get embedded inthe HTML. I don't see no `background` in the CSS. It causes me problems to extend the `<a>` tag to the heigh of the parent and adopt the bootstrap syntax.