Per instance stylesheets

We need a way to define different stylesheets per instance. The current way of adding CSS through the instance settings isn't sufficient as it only overrides previous settings and above all doesn't allow to use SASS.

This allows to declare a list of instance keys for which alternative stylesheets of the form `adhocracy_{key}.css` exist under `static/stylesheets`. Within such an instance that instance's css is loaded instead of `adhocracy.css`.

This extends the fanstatic resource mechanism by allowing to define dynamic resources, i.e. functions which are evaluated when needed. If we'd find a way to defer initialization until the adhocracy config is available, we could avoid this hack.

Note: Don't merge until the following task is done.

@xi: Can you add your sass-fu to that branch, which makes all files without underscores from `static_src/stylesheets` be compiled into `static/src`?