Fix nginx caching issues

Nginx is currently not caching data because proxy_buffering needs to be
enabled for caching to work at all, and we are receiving a Cache-Control
header from Pleroma that states "max-age=0, private, must-revalidate"

Even disregarding the Cache-Control header that should actually be set
to "public, max-age=1209600" as defined in the reverse_proxy code, we
don't want to obey this header at all as it overrides our Nginx caching
rules.