Try to fix the problem

Issue #15 (closed)

I tried to fix the problem (and I had a very hard time to realize that my inital idea on getting the old ids of the existing entries in the hierarchy setter failed due to wrong types (the ones in the db used ints, the ones from the hierarchy serializer used strings)).

I also changed some names, so that it is easier to see that they are many-relationships - and I added some cascade options.

Update 2020-11-11: I also changed the schemas a bit, to include the platforms & devices that used by the configuration in the api request.