Improve caching policy

* Cache css with version in url. This makes most js and css requests to
  be cached by the browser

* Force caching previews, the etag is in the url so that if the propfind
  gives a new etag, we will refresh it otherwise it's no use to try to
  fetch the new etag and do tons of DB queries

Tested with firefox and 'debug' => false (important so that the js/css
urls are generated with ?v= parameter)

Signed-off-by: Carl Schwan <carl@carlschwan.eu>