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
- [x] Add Django 5.0 support (it'll probably be released by then)
- [x] Drop Django 3.2 support