Cache the mirrorstatus json without potentially leaking secrets

[jelly]

The mirror status json endpoint behaves different if an authenticated user or normal user query it, making caching it return either the wrong data or sensitive data.

https://github.com/archlinux/archweb/commit/205ebb8f40982fe0c7b5b37af32b813ccaa012d2

[jelly]

Seems the django cache framework does not have an easy way to cache depending on the authenticated header, I've only found this decorator. As the main /mirrors/status/json feed is cached by nginx this issue's priority is low.