Introduce ElementCallView

Being in a call and deciding to show one are different jobs, but RoomPage
did both: routing, authentication, resolving room aliases and knocking, and
then the call itself. Only the first set belongs to whatever is hosting
Element Call.

Add ElementCallView as the seam between them. It takes a client and a call
to join, and owns whether the user has joined — which is the call's own
business rather than its host's. RoomPage keeps everything about arriving
at a call and renders this for the call itself.

Nothing else moves yet. muteStates is still passed in, because the
standalone shell shares one with the lobby it shows while waiting to be let
into a room, and two instances would both report the user's mute state to
the host.

src/room/RoomPage.tsx

@@ -25,7 +25,7 @@ import {
 import { useClientLegacy } from "../ClientContext";
 import { ErrorPage, FullScreenView, LoadingPage } from "../FullScreenView";
 import { RoomAuthView } from "./RoomAuthView";
-import { GroupCallView } from "./GroupCallView";
+import { ElementCallView } from "../ElementCallView";
 import { useRoomIdentifier, useUrlParams } from "../UrlParams";
 import { useRegisterPasswordlessUser } from "../auth/useRegisterPasswordlessUser";
 import { HomePage } from "../home/HomePage";
@@ -62,7 +62,6 @@ export const RoomPage: FC = (): ReactNode => {
   const { avatarUrl, displayName: userDisplayName } = useProfile(client);
 
   const groupCallState = useLoadGroupCall(client, roomIdOrAlias, viaServers);
-  const [joined, setJoined] = useState(false);
 
   const devices = useMediaDevices();
   const [muteStates, setMuteStates] = useState<MuteStates | null>(null);
@@ -125,11 +124,9 @@ export const RoomPage: FC = (): ReactNode => {
       case "loaded":
         return (
           muteStates && (
-            <GroupCallView
+            <ElementCallView
               client={client!}
               rtcSession={groupCallState.rtcSession}
-              joined={joined}
-              setJoined={setJoined}
               isPasswordlessUser={passwordlessUser}
               confineToRoom={confineToRoom}
               preload={preload}