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)