Record Identifier Model

[scaduto]

Closes #177

Renamed SourceSubstanceIdentifier Model to RecordIdentifier Model and attribute source_substance to record for clarity

[debboutr]

the identifier and identifier_label fields need to be required in this model, the blank and null args are confusing to model fields but for me after about the 47th time reading them it finally sank in.

In [47]: RecordIdentifier.objects.create(record=r, identifier_type=it)                                                                          
Out[47]: <RecordIdentifier: RecordIdentifier object (1)>

the above will be rejected in the serializer's validation process! and will result in an error

{
            "detail": "This field is required.",
            "status": "400",
            "source": {
                "pointer": "/data/attributes/identifier"
            },
            "code": "required"
        }