* extends 404 template (user should check the URL)
* introduces new decorator "uuid_required" which performs a check on a given 'uuid' or 'id' parameter
* throws a Http404 exception --> redirect to 404 template instead of 500 error template
* updates Django to 4.x and other packages (if possible) to latest versions
* Attention: Requires postgresql >= 12.0
* updates code fragments to match requirements of newer package versions