Create "Edit group details" widget

Allows you to change the

- Displayname
- Description
- Joinability. This can use the same style as the group visibility options instead of the style shown in the attached design

![groupeditdetailsoverlay](https://f.cloud.github.com/assets/109850/577866/d8d4e244-c842-11e2-9ed6-966692edaa6c.png)