Put always-on-screen widgets in top left

always-on-screen widgets now appear in the top-left where the
call preview normally is if you're not in the room that they're in.

Fixes https://github.com/vector-im/riot-web/issues/7007
Based off https://github.com/matrix-org/matrix-react-sdk/pull/2053

src/components/views/voip/CallPreview.js

@@ -1,5 +1,5 @@
 /*
-Copyright 2017 New Vector Ltd
+Copyright 2017, 2018 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -92,7 +92,8 @@ module.exports = React.createClass({
                 />
             );
         }
-        return null;
+        const PersistentApp = sdk.getComponent('elements.PersistentApp');
+        return <PersistentApp />;
     },
 });