Notifications for new issues.

Now notifies the relevant person about issues raised on datasets.  The definition
of 'relevant person' is:

- The contact-email on the dataset, or
- The contact-email for the publisher, or
- The system admin.

The admin email will only be used as the primary address when there is no other,
in which case it will simply be CC'd.
Generic fix for handling different group types outside of DGU