Thanks mcgeeco for reporting, and claudep and timgraham for review.

Forward port f19cadd3 from stable/1.10.x

Forward port 760f0767 from stable/1.10.x

Thanks Liz Lemon for the report and investigation and
Claude Paroz for the test.

This fixes some test failures on MySQL 5.7+.

Partially reverted refs #25532 to fix a regression in Django 1.10.
This reintroduces a crash for disabled forms.JSONField (refs #26949),
however, that issue is also present on Django 1.9.

Thanks Ryan Schave for the test.

Unused as of 2b039d96.