Show group detail breadcrumb link on edit page
Move group detail URL construction to model

- This property is idiomatic to Django
- Removes the need to add an additional context item
- The Django admin will create hyperlinks if this is defined
Correctly set group editability context var
Merge pull request #197 from azavea/feature/jcw/group-detail-link

Add group detail breadcrumb link to the top of group edit