Disentangle dependencies on other django packages

[fsbraun]

Currently djangocms-frontend depends on django-filer Some projects might want to use a different way of managing resources

The Tech Committee has voted on an approach @macolo proposed swappable models in https://github.com/django-cms/django-cms/discussions/7096 at least for django-filer files and images.

Additional context Of course it would be great if djangocms packages would follow the same recipe. (Who knows an existing example?)