Update private group sharing client to include a self-destruct timer in each message

This branch adds optional self-destruct timers to the INVITE, JOIN and LEAVE messages of the private group invitation protocol. The JOIN and LEAVE messages are used for manually accepting/declining invitations and also for automatically informing contacts that we've joined or left a group. Thus a JOIN or LEAVE message may be visible in the UI or not, depending on whether it was created manually or automatically. I suggest that we include a self-destruct timer in a JOIN/LEAVE message only if it's going to be visible in the UI. This will be implemented as part of #1832 (closed).

Closes #1831 (closed)