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/taskresults/tests/testutils.py

@@ -0,0 +1,21 @@
+from django.test import TestCase
+from django.test.utils import override_settings
+
+from taskresults.models import TaskResult
+
+
+@override_settings(
+    CELERY_ALWAYS_EAGER=True, CELERY_CACHE_BACKEND="memory", BROKER_BACKEND="memory"
+)
+class TestCaseWithCeleryTasks(TestCase):
+    def resetCeleryTasks(self):
+        TaskResult.objects.all().delete()
+
+    def assertCeleryTasksRun(self, tasks):
+        task_results = TaskResult.objects.all()
+
+        self.assertEqual(task_results.count(), sum([t[0] for t in tasks]))
+
+        creators = [r.creator for r in task_results]
+        for t_count, t_creator in tasks:
+            self.assertEqual(creators.count(t_creator), t_count)