Modernize React Redux

[Arnei]

Our redux code is still "oldschool", meaning it's unwieldy, there's a hell of a lot of boilerplate and it's nearly impossible to type well. Therefore we should try to migrate to "modern" redux, using redux-toolkit.

Also recommended by the developers of React Redux: https://redux.js.org/usage/migrating-to-modern-redux

[Arnei]

#214 did not fully resolve this issue, so reopening. This is one of those issues that gets resolved over multiple PRs anyway. #214 does however serve as a reference on how to further resolve this issue.