[SPIKE] Isolate firebase dependency

[gesellc]

for discussion, not necessarily to be merged right away

[sturmf]

Hmm I am not sure about those methods. I think they only exist for the Firebase usecase. E.g. if we would have a databse or file as a backend, we would do the saving to disk and dispatching the success in a single method without the need of an async callback.

[gesellc]

Currently those are commented out and only introduced in the FirebaseService Class. But mostly because I didn't find a way to make it work as a abstract class/implementation pair.

My reasoning to have those in the PercistencyService Class was that the frontend currently assumes async communication with the persistency layer - specifically in the action design. I would then assume that we need some sort of success messages flowing back to the frontend.

[gesellc]

The initial reason of defining those in the PercistencyService Class was to decouple the usage of Redux in the frontend from the usage of firebase in the backend.

If this goal is even valid might also need some discussion... :-)