Remove unnecessary CSS import

docs isn't actually needed for docs large - it needs either docs or
base. And those are always loaded before docs_large anyway.
Dynamically merge CSSes into a single one

This will make CSS downloading a single request, instead of making
6-7 requests for each page as it is now. It also moves all the definitions
of URLs for CSS into the templates and not in the raw CSS itself, which
will make it possible to enable client side caching in the future.

Fixes #91