Tech Stack: Use redux toolkit

[NearW]

Sine we already have a global event based state with a reducer, it makes sense to use redux toolkit over our custom context solution.

https://redux.js.org/tutorials/fundamentals/part-5-ui-react

[dostulataa]

https://github.com/MaibornWolff/retro/tree/tech/209/introduce_redux_toolkit

I created the slices for the retro state and its store. This still needs 1) linking to the ui and 2) some solution on how to sync the state to the session's state for a user that is joining.