include all styles and js and cache header

We don't have any as_needed styles or scripts that would be worth excluding.

Caching the head saves ~25 ms on each request. Not that much but still more than the 5 ms the data transfer of the entire css file takes. Further more these will be cached on the client anyway.
cache the login form, current_language_code helper
improved top menu caching
add cache_digests and use it for the top menu
move permission migration test out of main user test
cleanup and test user and group versioning
fragment caching basics

* current_theme now responds_to cache_key
* users and groups have two cache keys now:
  - version_cache_key, bumped when their associations change
  - cache_key, bumbep when they themselves change
  - login link on top will always be rendered
use new cache key format all over the place
deprecated: new syntax for word_wrap and table_name=
fix assets: move css styles, add application.js