Make it possible to change the default AutoField to BigAutoField.

[Caio Ariede]

This was initially proposed in this comment/PR: ​https://github.com/django/django/pull/8924#issuecomment-516792989

1. I really like the proposed approach of having a DEFAULT_AUTOFIELD setting.
2. For backwards compatibility, when DEFAULT_AUTOFIELD is not set, we should keep the default behavior: django.db.models.AutoField
3. Whenever this setting is changed, it would affect all existing AND new fields (pk and fk)

[Mariusz Felisiak]

I think we need a discussion and consensus on the DevelopersMailingList before we move forward with this ticket. Adding a new setting is always a bit controversial and changing the "default" AutoField for all tables is a complicated and error-prone task. That's why we need a clear path, I'm closing before we finish a discussion.

[Tom Forbes]

Proposal: ​https://groups.google.com/d/msg/django-developers/MBPEPKlibGQ/F4i6JPvBAAAJ

[Tom Forbes]

Re-opening as per the mailing list discussion where we have broad consensus, and a good two-phase plan of action.

[Tom Forbes]

PR: ​https://github.com/django/django/pull/13179

[Tom Forbes]

Addressed review comments and improved the implementation a bit.

[Mariusz Felisiak <felisiak.mariusz@…>]

In b5e12d49:

Fixed #31007 -- Allowed specifying type of auto-created primary keys.

This also changes the default type of auto-created primary keys
for new apps and projects to BigAutoField.

[Mariusz Felisiak <felisiak.mariusz@…>]

In f07723aa:

Refs #31007 -- Added test for check for specifying type of auto-created primary keys from abstract models.