Support multiple integration managers behind a labs flag

Fixes https://github.com/vector-im/riot-web/issues/10622
Implements [MSC1957](https://github.com/matrix-org/matrix-doc/pull/1957)

Design is not final.

src/components/views/settings/IntegrationsManager.js

@@ -43,7 +43,7 @@ export default class IntegrationsManager extends React.Component {
         configured: true,
         connected: true,
         loading: false,
-    }
+    };
 
     componentDidMount() {
         this.dispatcherRef = dis.register(this.onAction);