Add basic group detail page

The rough layout was inspired by the wireframes.
Add a form to edit group settings

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.
Merge pull request #112 from jwalgran/topic/group-detail-page-basics

Group detail page basics