Start the migration to v1 VoIP by supporting m.call.reject, which
we'll send if the caller says they're v1. Our version stays as v0
for now, until we speak the rest of v1.
Honour the default reaosn in a hangup being user_hangup.
As per comment, this seems to be the way to make setSinkId work reliably on Chrome.
Also don't make our own mediastreams if we don't need to (ie. in a normal call
our tracks should come with their owen MediaStreams).
This does a fairly basic rewriting of the whole file to bring it up
to a vaguely modern standard of code. Nothing should have changed
fundamentally, but it probably should do: as per the global type
overrides, we use a lot of deprecated WebRTC APIs and should probably
update.
David Baker