Automatically decline incoming private group invitations when they self-destruct
Test instructions:
- use two or more contacts
- create various private groups
- invite contacts to private groups
- accept/decline invitations
- enable/disable disappearing messages throughout the process
- ensure that messages sent with enabled timers self-destruct (also while private conversation is open)
- ensure that open invitations get automatically declined and are recognizable as automatic for the sender of the decline
Does implement #1836 (closed)