common: context manager for activities

circle/common/models.py

 from collections import deque
+from contextlib import contextmanager
 from hashlib import sha224
 from logging import getLogger
 from time import time
@@ -32,6 +33,9 @@ def activitycontextimpl(act, on_abort=None, on_commit=None):
         act.finish(succeeded=True, event_handler=on_commit)
 
 
+activity_context = contextmanager(activitycontextimpl)
+
+
 class ActivityModel(TimeStampedModel):
     activity_code = CharField(max_length=100, verbose_name=_('activity code'))
     parent = ForeignKey('self', blank=True, null=True, related_name='children')