Show open button in private conversation after accepting invitations

To keep the implementation simple, the Open button does appear where the Accept button had been previously.

In order to make the Open button functional, I had to make the `GroupId` of the invitation target available to the UI. Most code in this MR is due to that.

![device-2016-12-13-105228](/uploads/d0f27fc02f1411596458d9203a0810d2/device-2016-12-13-105228.png)

Closes #793

See merge request !457