Crib comment creation fails if contributed crib has no associated user.

Sentry Issue: STRATHSPEY-15

AttributeError: 'NoneType' object has no attribute 'email'
(2 additional frame(s) were not displayed)
...
  File "ace4/db/views/issues.py", line 242, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "ace4/db/views/issues.py", line 190, in form_valid
    recipient = self.object.user.email