Support arbitrary widgets sticking to the screen + sending stickers

Following https://github.com/matrix-org/matrix-react-sdk/pull/5385, it is now possible for a widget to request these capabilities without being a video conference or sticker picker. This commit actually enables this support for those kinds of widgets.

This commit also fixes an issue in the URL templating where some variables might get set to 'undefined' - this appears to be a scoping issue, so StopGapWidget now stores the definition alongside the superclass. 

Fixes https://github.com/vector-im/element-web/issues/15001

src/components/views/elements/PersistentApp.js

@@ -71,7 +71,6 @@ export default class PersistentApp extends React.Component {
                     appEvent.getStateKey(), appEvent.getContent(), appEvent.getSender(),
                     persistentWidgetInRoomId, appEvent.getId(),
                 );
-                const capWhitelist = WidgetUtils.getCapWhitelistForAppTypeInRoomId(app.type, persistentWidgetInRoomId);
                 const AppTile = sdk.getComponent('elements.AppTile');
                 return <AppTile
                     key={app.id}
@@ -82,7 +81,6 @@ export default class PersistentApp extends React.Component {
                     creatorUserId={app.creatorUserId}
                     widgetPageTitle={WidgetUtils.getWidgetDataTitle(app)}
                     waitForIframeLoad={app.waitForIframeLoad}
-                    whitelistCapabilities={capWhitelist}
                     miniMode={true}
                     showMenubar={false}
                 />;