Implement call hold

Currently just by adding /holdcall and /unholdcall slash commands

The only place the hold status of the call is currently represented
is when the call is a voice call and you're viewing a different room:
it's not wired up when you're viewing the room because that currently
uses the room status bar which it won't do with the new UI.

Also convert VideoFeed to typescript, and remove videoview because
it essentially just managed the fullscreen functionality, but we'll
want and 'on hold' representation (and probably chrome for hagnup etc)
in the fullscreen UI too, so let's just make CallView the thing that
gets fullscreened.

src/@types/global.d.ts

@@ -65,6 +65,13 @@ declare global {
     interface Document {
         // https://developer.mozilla.org/en-US/docs/Web/API/Document/hasStorageAccess
         hasStorageAccess?: () => Promise<boolean>;
+
+        // Safari & IE11 only have this prefixed: we used prefixed versions
+        // previously so let's continue to support them for now
+        webkitExitFullscreen(): Promise<void>;
+        msExitFullscreen(): Promise<void>;
+        readonly webkitFullscreenElement: Element | null;
+        readonly msFullscreenElement: Element | null;
     }
 
     interface Navigator {
@@ -94,4 +101,11 @@ declare global {
     interface HTMLAudioElement {
         type?: string;
     }
+
+    interface Element {
+        // Safari & IE11 only have this prefixed: we used prefixed versions
+        // previously so let's continue to support them for now
+        webkitRequestFullScreen(options?: FullscreenOptions): Promise<void>;
+        msRequestFullscreen(options?: FullscreenOptions): Promise<void>;
+    }
 }