v8 Django model field improvements

[jacobtylerwalls]

#11596 used some features on Django model fields that we haven't always used, e.g.:

• blank
• default
• related_name
• related_query_name

As well as some logical constraints that can be enforced in Postgres (and validated with Django's model validation framework). Never mind, I'll do that in a separate PR.

This ticket is to break out these improvements from #11596 and discuss them separately.