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.

Send party_id on events and check the party_id of incoming events matches

Includes a basic test to assert that it actually does: we should
build out a decent test suite for calls as there's a lot of edge-case
functionality that can break and slip through the cracks (eg. glare).
This is a start.

Fixes #1511

Typescript tests basically just appear to work, apart from needing
the jest types imported so the typescript checker knows what's what.

DConvert the webrtc test to typescript, which actually mostly just
serves to point out that we're not mocking the whole of `document`,
but oh well.