Refactor managemails to use signals

- use signals to trigger Celery tasks to create and delete mailboxes
- add generic TestCaseWithCeleryTasks class to handle celery task tests
  in a uniform way

gnuviechadmin/managemails/apps.py

@@ -15,3 +15,11 @@ class ManageMailsAppConfig(AppConfig):
 
     name = "managemails"
     verbose_name = _("Mailboxes and Mail Addresses")
+
+    def ready(self):
+        """
+        Takes care of importing the signal handlers of the :py:mod:`userdbs`
+        app.
+
+        """
+        import managemails.signals  # NOQA