Clean up project for 1.0 release

[sergei-maertens]

Let's make the 1.0 release a neat and tidy one.

Tasks

• Delete legacy cookiebar (static/cookiebar.js)
• Delete deprecated template tags
• Check code files for style & make them pass flake8
• Check which functionality may be obsolete
• Add Django 5.0 support (it'll probably be released by then)
• Drop Django 3.2 support