It's a transitional setting helpful in migrating multiple instance of
the same project to Django 3.1+.
Thanks Markus Holtermann for the report and review, Florian
Apolloner for the implementation idea and review, and Carlton Gibson
for the review.
Backport of d907371ef99a1e4ca6bc1660f57d81f265750984 from master.
This is the new contract since middleware refactoring in Django 1.10.
Co-authored-by: Carlton Gibson <carlton.gibson@noumenal.es>
Co-authored-by: Mariusz Felisiak <felisiak.mariusz@gmail.com>
