Introdcues a new update service.
Listens to service-worker updates and shows a snack-bar to inform about updates.
Provides a function to manually check for updates.
The service worker tries to be consistent in it's own version and
updates in the background.
Some manuall trigger will be required to update, which is either a
reload or the execution of the provded check function
with help from
@FinnStutzenstein
* Activate restricted_data_cache on inmemory cache
* Use ElementCache in rest-api get requests
* Get requests on the restapi return 404 when the user has no permission
* Added async function for has_perm and in_some_groups
* changed Cachable.get_restricted_data to be an ansync function
* rewrote required_user_system
* changed default implementation of access_permission.check_permission to
check a given permission or check if anonymous is enabled
