Prepare for Django 4.0

Use scrypt password hasher
Use native Redis cache backend
Use FormMixin API of DeleteView (@hansegucker, please check: https://docs.djangoproject.com/en/dev/releases/4.0/#deleteview-changes)
Suppot settingtrusted origins (drive-by fixing missing feature)
Remove leading / from STATIC_URL (drive-by fix)
Remove USE_L10N and USE_I18N from settings as they now default to True

Edited Dec 11, 2021 by Jonathan Weth