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.
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Backport of 240cbb63bf9965c63d7a3cc9032f91410f414d46 from master