This removes the dependency of the Index class on its 'model'
attribute.

Thanks marktranchant for the report and review.

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.