boostrap 5 (version 3)

This is based on previous attempts in !1719 (closed) and !1884 (closed) as well as the official migration guide.

I took a somewhat unconventional approach: I created a script that does the grunt work of the migration and then deleted itself. That should ease much of the pain of rebasing this MR on top of future changes.

The major issue in !1884 (closed) was the missing support in fullcalendar. Based on the discussion in the fullcalendar repo I figured it could be so hard and made some changes myself (see d8e930e4).

Unfortunately I had to monkeypatch django-bootstrap5. Maybe that is my personal taste, but I really don't like all fields turning green. There would also be the option to set 'server_side_validation': False, but that would also remove some is-invalid classes that we use in our tests (and that are actually helpful).

I think it would be nice to get this into a release together with all the UI changes so we have a shiny new UI for the new year.