Implement parts of the frontend with React native for the moble apps

[joergreichert]

Current state: the app is only integrated as web view, so even the sidebar is still web.

Motivation: The sidebar doesn't look / behave as you would expect it by a mobile app.

Future state: There is switch in the app, that detects, that is running in mobile app context and thus uses the React native implementation of the sidebar, that feels more appropriate.