Implement UX for showing and answering private group invitations

As usual, this MR contains several logically separate commits that could be split out into smaller MRs if desired. It consists of two main parts:
* Showing open invitations in the list of private groups with a snackbar
* Showing invitations and responses in the private conversation

For both parts, the existing code was refactored to allow for a smooth implementation and to leave maintainable code behind.

![device-2016-10-18-101549](/uploads/66582dbe97736fdcd2498e87e1c7dfd1/device-2016-10-18-101549.png)
![device-2016-10-18-101612](/uploads/8c25eff8171f330796a55cb27cdb2552/device-2016-10-18-101612.png)
![device-2016-10-18-101534](/uploads/ebba4c0a2c0f727dcadac8c2ec57b48f/device-2016-10-18-101534.png)

Closes #707

See merge request !357