- use signals to trigger Celery tasks to create and delete mailboxes
- add generic TestCaseWithCeleryTasks class to handle celery task tests
in a uniform way
This commit adds tests for managemails.models to improve the test
coverage of that model. There are some changes to the classes in
managemails.models too:
- add a method create_mailbox to MailboxManager
- properly handle uncommited mailaddresses in MailAddress.set_mailbox
and MailAddress.set_forward_addresses