Allow serving static pages from the database

Currently, the static pages are hardcoded. That means that administrators have to fork their own style just to change common stuff like the imprint. This pull request alleviates the issue by adding an `adhocracy.staticpage_backend` option that can be set to `filesystem` (default, old behavior) or `database`. If it is set to the latter, global administrators can change imprint etc. without modifying code.

Our issue for this is hhucn/adhocracy.hhu_theme#96 .