Group detail page basics

This pull implements basic view and edit functionality for Groups.

The rough layout was inspired by the wireframes.

I followed an often used convention of using ``/model/{id}/edit/`` as the url for displaying the form. The form POSTs back to ``/model/{id}/edit/`` and then redirects to ``/model/{id}/`` on success.
Consider changing this to `<div>{{group.contact_info}} {{ group.contact_email|urlize }}</div>`

Seeing the email address makes it more clear that this is a `mailto:` link, and not a web-based contact form.