Serializer classes for model <-> json api object conversion

In order to reduce the amount & the complexity of code changes in the future - and to improve test ability - I propose that we introduce serializer classes for the conversion between the models and the json api objects.

That way we can write more tests for the conversion, which is especially helpful for changes on the backend site.

This here can be part of a larger refactoring of the service classes, that may can split abstraction levels into several components (service (high level), controller (mid-level, does the web requests) & serializer (just the conversion).