Merge branch '398-forum-sharing-status' into 'master'

Forum Sharing Status

The new activity shows who you are sharing a forum with and who shares a
forum with you. It is accessible from the overflow menu when in a forum.

![device-2016-05-13-153259](/uploads/2e1c651bc72094018dd0f97a37453d1b/device-2016-05-13-153259.png)

Closes #398

See merge request !191