Use the 'reject' method when we want to reject an incoming call
rather than end one that's in progress. Also get our error messages
right for the other side rejecting the call (albeit still with
placeholder dialog-box UX).
Requires https://github.com/matrix-org/matrix-js-sdk/pull/1510
* Remove the two separate enumerations of call state: now everything
uses the js-sdk version of call state. Stop adding a separate
'call_state' field onto the call object(!)
* Better reflection of the actual state of the call in the call bar,
so when it's connecting, it says connecting, and only says 'active call'
when the call is actually active.
* More typey goodness
and remove the old conference call stuff while we're at it: enough
time should have passed since those mistakes that we can move on.
The old conference call rooms will still appear for anyone whose
account dates back to that time, but they've presumably been appearing
in any other matrix client they used too.
