Cache different versions of /downloads for Django users

[hugovk]

Description

• Follow on from python/pythondotorg#2843
• I can see the edit button locally on http://0.0.0.0:8000/downloads/release/python-3142/ but not at https://www.python.org/downloads/release/python-3142/
• I can see the edit button on pages such as https://www.python.org/psf/developersinresidence/ have an edit button
• I think this is because Fastly is caching /downloads for all users in the same way, because the Django Vary header is being hidden for /downloads but not other pages
• This caching was added in python/pythondotorg#1390 along with other measures, and the majority of visits are to /downloads
• Perhaps it's safe to expose Vary and keep the other measures?

[JacobCoffee]

i think this might be better done in fastly?

[JacobCoffee]

if i understand correctly, what about #2850

[hugovk]

Yeah, Fastly sounds better, but I'm not too familiar with its config to review it.