Forwardport of 5b3c66d8 from stable/1.8.x

Thanks to Karl Hobley for the report and Markus, Shai, Aymeric for their input
and Tim for the review.

Thanks andreif for the contributing to the patch.

There is no BCryptSHA512PasswordHasher.

Follow up to refs #25997 but this case wasn't broken.

Because we now load data into the page via JSON, we don't need to
unescape it anymore.

Thanks Tim Graham for the review.

Fixed #26034 -- Fixed incorrect index handling on PostgreSQL on Char/TextField with unique=True and db_index=True.

Thanks Simon Charette for review.

Forwardport of test in o839d71d8562abe0b245024e55ca1d02a45e58fd from stable/1.9.x
(refs #25997).

Thanks Thomas Grainger for the fix.

This reverts commit 312fc1af as it seems
to cause segmentation faults as described in the ticket.

Thanks to Dave Voutila for the report and Tim for the review.

Added an optional `backend` argument to login().

Now intermediate states in the database_backwards are cached, similar to
the executor's migrate() (or _migrate_all_backwards()). This makes the
migration run much faster (O(n) instead of O(n^2) over number of
database_operations).