Add blog sharing information to toolbar subtitle

This MR introduces a new `SharingController` which is supposed to be used in activities that show blogs, groups and forums. Feedback on this approach is welcome before this is used to add "Shared with" support to other parts of the UI.

The toolbar subtitle shows information about how many contacts the current shareable is shared with and how many of those are online.

So far this is implemented for blogs:

![device-2016-12-05-180207](/uploads/13afb4c7c5d10ad29c414865ee02670d/device-2016-12-05-180207.png)

One part of #646

See merge request !447