Fix mongoID field's max length.
Implement partially broken deserialization.

This may or may not work with the `CQRSSerializer(data=…)` technique; I
haven't tried it at all.

Unfortunately it appears that somewhere, deep in the bowels of Django
REST framework, probably, the `mongoID` field is lost in
deserialization. Hence the test_c_deserialize and test_d_deserialize
tests fail. To me this is inexplicable; I've tried somewhat to
understand it or to work around it, but in vain.
Refactor registration stuff out of serializers.

This is so it can be conveniently used for document subcollections in
Mongo, which I'm just about to do.