custom model field validators need serialization ability

[bengolder]

I tried to make new migrations for the application transfer, and ran into an error related to the TemplateFieldValidator

  File "/Users/bengolder/projects/intake/lib/python3.5/site-packages/django/db/migrations/serializer.py", line 388, in serializer_factory
    "topics/migrations/#migration-serializing" % (value, get_docs_version())
ValueError: Cannot serialize: <intake.validators.TemplateFieldValidator object at 0x105745cc0>
There are some values Django cannot serialize into migration files.
For more, see https://docs.djangoproject.com/en/1.10/topics/migrations/#migration-serializing

I found a related issue: https://code.djangoproject.com/ticket/21275
And this commit shows some examples of designating validators as serializable:
loic/django@92e9e0c