Discussion of proposed design for careteam work

Available for review, but please don't merge yet.
The /confirm/invitations call is asking the question "which are MY invitations?", which is the only call that identifies the receiver rather than the sender, which is why it's the one that's different.

The "it's a PUT" comment has been deleted -- there were server implementations once that actually forbid the use of a body with a PUT and my understanding of PUT was incomplete.