Allow more control over recipients of message encryption keys

In some situations one does not want all room members to be able to read one's messages:
- In a reporting room, several people could each privately report to the room admin.
- For a party, people might want to report what they bought for whom - to all but the gift recipient.
- When playing diplomacy, this avoids having to create n² rooms for all the subsets of the players.
A nice interface would be nice, both on the sending and on the receiving side: one wouldn't want lots of "The server has sent no keys for this message - request them from your other devices" garbage.

No files, tests, or entry points are named. Start by clarifying the sender and recipient-selection semantics for encrypted messages, including how excluded recipients should avoid missing-key warnings. Done should include agreed sending and receiving interfaces that support the reporting, gift, and diplomacy cases.

Written by the indexing model from the issue text.