fix empty database migrations

[shapiromatron]

It's been a long time since we started a fresh version of hawc, and we had few issues where if you run django database migrations from scratch, it didn't work.

1. Fixed an error where a batch_size is now required when using prefetch_related and iterator
2. Fixed an issue where a vocab fixture attempted to load a deprecated field, before the deprecated field was created.

To check, update your local.py settings to use a database named foo, and then:

dropdb -U hawc foo
createdb -U hawc foo
python manage.py migrate

[caseyhans]

Nice! Works like a charm.