I've finally finished my new #Django module, "django-cool-urls". You can use it to make sure your site never has another outgoing dead link, or just to convert YouTube embeds into proper <video> tags pointing to local files.
Explanatory blog post: https://danielquinn.org/blog/django-cool-urls/
PyPI: https://pypi.org/project/django-cool-urls/
Source code: https://gitlab.com/danielquinn/django-cool-urls
Docs: https://danielquinn.gitlab.io/django-cool-urls/
It's already in use on my own site, but perhaps someone else will find it useful.