Support rejecting calls

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

src/components/views/voip/IncomingCallBox.tsx

@@ -78,7 +78,7 @@ export default class IncomingCallBox extends React.Component<IProps, IState> {
     private onRejectClick: React.MouseEventHandler = (e) => {
         e.stopPropagation();
         dis.dispatch({
-            action: 'hangup',
+            action: 'reject',
             room_id: this.state.incomingCall.roomId,
         });
     };