Allow callers to cancel ringing calls

[eamsden]

switchboard

Check implementation:

• Allow %reject pokes from airlocks to produce %hungup pokes on %ringing connection-state calls
• Produce %incoming-call-hangup fact for incoming calls

urchatfm

Send %reject poke when call is in %ringing state and we click the 'Hangup' button

This may already work as implemented, but requires testing.

[eamsden]

this should work but should be better tested. In particular closing local video streams should still happen in this case @arthyn