Add group detail breadcrumb link to the top of group edit

Fixes #176 
It might be a better way to handle this by defining `get_absolute_url` on the model.

Pros:
- it's idiomatic to django
- encapsulation - info about a model, on the model
- you don't have to add another attribute to the context
- we'll get a link to the detail page for free in the admin interface (!)

Cons:
- Slightly more hidden logic in the template
- encapsulation - the (lower) model layer knows about the (higher) url/routing layer

https://docs.djangoproject.com/en/1.7/ref/models/instances/#get-absolute-url