Update application code to use Django 3.x

[KHIT93]

We have been waiting with upgrading to Django 3.x as some of the packages used, were not yet compatible.

We have decided to remove the current audit logging functionality, which was provided by django-auditlog

Additionally, we have retired django-encrypted-model-fields, as it had some issues with the generated encryption key on new installs. We have instead using django-cryptography instead.

We will need to provide a migration path between the two libraries, as users with 2FA enabled, will loose the configuration.
If a valid migration path cannot be found, then users would need to re-enable 2FA on their account

With this change, we are also changing some of the terminology used, to better align with MailScanner.
See here for related information: MailScanner/v5#492