detect safety number changes from incoming messages

STUB

• currently we can only detect changing safety numbers when we fail to send a message to a user
• among other things, this makes it possible for an admin to be locked out without knowing it (and in a way that is not discoverable until other admins send a message)
• it would be nice if we could detect safety number changes when a user with a new safety number sends an incoming message, but that is blocked on needed changes to the signald api. (see: https://gitlab.com/thefinn93/signald/issues/4#note_265584999)
• this card would make those upstream changes and adopt our usage of the API accordingly)