- use signals to trigger Celery tasks to create and delete mailboxes
- add generic TestCaseWithCeleryTasks class to handle celery task tests
in a uniform way
- don't execute celery tasks directly
- introduce optional parameters to fileserver tasks to allow chaining
- handle user/group/key create and delete tasks in new osusers.signals
class
- adapt unit tests
- change TaskResults model to store the task signatures
- generalize the local settings' logging configuration
