Show list of attached datasets on data group/type admin pages.

Use admin.StackedInline to show a list of attached datasets on
admin pages for datagroups and datatypes.

These are read-only, and datasets cannot be deleted from these
pages. However, new datasets can be added here - we can disable
this if we want.
Merge pull request #113 from alphagov/show-attached-datasets-in-admin

Show list of attached datasets on data group/type admin pages.