We're seeing a couple of final quirks running the testsuite on django
1.6:

  Traceback (most recent call last):
    File "patchwork/apps/patchwork/tests/notifications.py", line 182, in testNotificationEscaping
      errors = send_notifications()
    File "patchwork/apps/patchwork/utils.py", line 227, in send_notifications
      delete_notifications()
    File "patchwork/apps/patchwork/utils.py", line 197, in delete_notifications
      pk__in = notifications).delete()
    File "/usr/lib/python2.7/dist-packages/django/db/models/manager.py", line 163, in filter
      return self.get_queryset().filter(*args, **kwargs)
    File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 590, in filter
      return self._filter_or_exclude(False, *args, **kwargs)
    File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 608, in _filter_or_exclude
      clone.query.add_q(Q(*args, **kwargs))
    File "/usr/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1198, in add_q
      clause = self._add_q(where_part, used_aliases)
    File "/usr/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1232, in _add_q
      current_negated=current_negated)
    File "/usr/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1122, in build_filter
      lookup_type, value)
    File "/usr/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 1107, in get_lookup_constraint
      values = [get_normalized_value(value) for value in raw_value]
    File "/usr/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 1084, in get_normalized_value
      value_list.append(getattr(value, source.attname))
  AttributeError: 'PatchChangeNotification' object has no attribute 'id'

- we're specifying our own pk here, so the PatchChangeNotification has
no id attribute; it looks like the pk__in syntax is expecting IDs.

We also need a default value for BooleanField, as we're getting
integrity errors when creating rows with no explicit send_notifications
set.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>