Allow dynamic fanstatic resources

This allows to create fanstatic resources during runtime.

This is pretty ugly and wouldn't be needed if we could defer
initialization of the fanstatic resources until the adhocracy config is
available.
Allow to declare instance stylesheets

This allows to declare a list of instance keys for which alternative
stylesheets of the form `adhocracy_{key}.css` exist under
`static/stylesheets`.

If a page within such an instance that instance's css is loaded instead
of `adhocracy.css`.
Merge branch 'per-instance-css' into develop (fixes #475)