Refactor the way group calls hang up (#3234)

* Refactor how group call end calls

We previously used disposeCall to terminate the call which meant that
sometimes a call would never get a hangup event. This changes it so
that we always end a call by calling hangup, then do the cleanup
when the hangup event arrives, so the cleanup is the same whether
we hang up or the other side does.

* Some fixes for failing & hanging tests

* Add type for the call map

spec/test-utils/webrtc.ts

@@ -511,6 +511,8 @@ export class MockMatrixCall extends TypedEventEmitter<CallEvent, CallEventHandle
     public callId = "1";
     public localUsermediaFeed = {
         setAudioVideoMuted: jest.fn<void, [boolean, boolean]>(),
+        isAudioMuted: jest.fn().mockReturnValue(false),
+        isVideoMuted: jest.fn().mockReturnValue(false),
         stream: new MockMediaStream("stream"),
     } as unknown as CallFeed;
     public remoteUsermediaFeed?: CallFeed;