AvailableMeasurementTypes now have an "implementation". Several types
may share the same implementation, and thus do the exact same
thing. If no implementation is given, the "slug" field is used, so
that for types for which this feature isn't used, nothing changes.
We can now configure which AvailableMeasurementTypes are allowed for
each organization. In the same models, the organizations will be able
to say which of those they want to be visible.