Fixed #23587 -- Clarified admin template overriding in tutorial 2.

Thanks Petrus van Bork for the report.
Added 1.7.1 release notes for refs #23333.
Fixed #23555 -- Avoided suppressing IndexError in QuerySet.first() and .last()
Fixed typos in comments.
Fixed #23600 -- Made default_storage aware of more settings changes.

Added MEDIA_URL, FILE_UPLOAD_PERMISSIONS, and
FILE_UPLOAD_DIRECTORY_PERMISSIONS to the list of settings.
Fixed flake8 warning.
Fixed #23609 -- Changed behavior when dropping a NULL constraint to update rows with NULL to default
Referred #23609 -- Added migration questioner support for NOT NULL alterations