replace django-bootstrap4 by django-bs4

A while ago I got annoyed with django-bootstrap4 because of issues like !1405 (merged), so I re-implemented the whole thing and called it django-bs4. Now I finally found the time to test how well this works in the context of castellum.

I designed it as a drop-in replacement, so no changes are required to the code. As far as I could see this works just fine.

For the most part, django-bootstrap4 is great. We already have workarounds for the file widget. So I would not merging using this MR. But it is nice to know we have the option.