[WIP] partial port to bootstrap

Currently breaks everything (main page).
Partial port of header to bootstrap. Improvement of display on little screens.
Help required.
We are using [sprites](https://en.wikipedia.org/wiki/Sprite_%28computer_graphics%29#Sprites_by_CSS) and the background image comes from the class `icons-notifications_grey` which sets `app/assets/images/icons/notifications_grey.png` as the background for this div. (the real background that it sets is `app/assets/images/icons-********.png` with some offset.)

Instead of those you could also use entypo icons (see #5283). The icons @roymckenzie used in that issue (`<i class="entypo mail"></i>` and `<i class="entypo bell"></i>`) would be ok for me but others may have a different opinion, so be prepared for some discussion. ;-)