[FE] - introduce architecture for state management

[mburri]

There is currently no state management library used in the frontend. The application service (app.service.ts) is capable of holding global state - but there is no clear architecture nor a guide on how to use this.

TODO:

• decide if it makes sense to use a state management library for liima
  • candidates could be:
    • ngrx
    • ngxs
    • angular-redux
• or if we go with bulid-in angular features - (rxjs)
  • advantage: no additional dependecies needed.

I'd go for the last approach and clean up the current "mess"

[mburri]

see https://dev.to/avatsaev/simple-state-management-in-angular-with-only-services-and-rxjs-41p8

It's not obvious if Liima could benefit from a State Management Lib - or if one is needed...