Implement UX for viewing the membership of a private group

This MR is the second and last MR to address #678. The first part is in !377.

![device-2016-10-26-112000](/uploads/8cbdee65c123a6d5329e208d9983d0b0/device-2016-10-26-112000.png)

Closes #678

See merge request !364