Always set X-Forwarded-Proto to avoid mixed content warnings

This fixes mixed content warnings I've been getting when loading album covers. As documented in funkwhale#901 (closed) and https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/237 , the X-Forwarded-Proto header needs to be set to https so album covers will be loaded securely, even when we have a TLS terminating proxy.