refactor: extract functions in playground

[slettner]

The playground.py app is growing with more apps being added now. We can restructure it a bit and extract some code that is modality specific while removing duplication for common logic.

The output would look similar to the structure of the backend with code being separated in app/modalities.

Another improvement would be to use the constants and app configs from the now library in the playground app.

cc @florian-hoenicke @joschkabraun