Breakup serializers.py

serializers.py file has become too fat and cluttered and hard to navigate. It's time to break it up into multiple files.

Serializers now:

opdm-service/
|-- ...
|-- project/
    |-- __init__.py
    |-- ...
    |-- opdm_service
        |-- __init__.py
        |-- ...
        |-- serializers.py

How they should be:

opdm-service/
|-- ...
|-- project/
    |-- __init__.py
    |-- ...
    |-- opdm_service
        |-- __init__.py
        |-- ...
        |-- serializers
            |-- __init__.py
            |-- area.py
            |-- person.py
            |-- organization.py
            |-- <model_name>.py