Django 3 migration

This merge request is the base of the migration of beat/beat.web> to Django 3.

Preliminary work includes:

• Fix all applications with regard to the namespace handling
• Fix all missing on_delete parameter in both models and migration
• Removal of obvious Python 2 code path
• Example fix of URL_PREFIX abuse
• Port of login handling to new view system
• Update call to function that have become properties
• Update to imports

Related issues:

Fixes #561 (closed)

Fixes #559 (closed)

Part of #558 (closed)