Add sharing info to ForumActivity action bar subtitle

This indirectly does also most of the work for adding the same information to private groups. However, completing this is blocked by !448.

![device-2016-12-07-152915](/uploads/5bb42f2e78a87931b0307e7597ba72c5/device-2016-12-07-152915.png)

Second part of #646

See merge request !451