Merge branch '461-forum-invitation-can-not-be-accepted-if-forum-already-added' into 'master'

Show all Forum Invitations, not only the first one to prevent cut-offs and forks

This MR solves the problems outlined in #461 by always showing forum invitations, even if the user already subscribed to the forum.

Available Forums have been renamed to Forum Invitations in the UI:

![device-2016-07-07-144141](/uploads/9c8d06decdd19117250e3faca0c38192/device-2016-07-07-144141.png)
![device-2016-07-07-144150](/uploads/9803f567c962a721b24d79359974a29f/device-2016-07-07-144150.png)

Closes #461

See merge request !241