When accepting an invite for a meeting where the invitee already has access, a permission error is shown

[robinharms]

Things already work though, but it looks like something has gone wrong.
Investigate also what happens when you go to URL of meeting you're not in yet. (should redirect to login or join)

[schyffel]

Work done in 4962d3d

[schyffel]

Depends on VoteIT/voteit#344