New minimigration PR, just to take out all old formatting and make it all-bootstrap

It will take some js-magic too, because there is a lot of Prototype left. In the end the targets are:

* to have almost no app-specific css (use `bootstrap` plus `font-awesome` and for the missing parts, base on compass. This leads to a lesser weight (as bootstrap and font-awesome are served from CDN) and less maintenance overhead.
* to delegate that most JavaScript magic is from either `rails-ujs` (using `data-*` attributes) or vendored jquery plugins (i.e., bootstrap's tabs and tooltips, select2, timeago...)
* to have cleaner ERB templates, avoiding messy helpers in favour of explicit HTML and templates.