/feed doesn't use protocol of front end server

[tehmoth]

when proxying to an instance of towncrier from an https website, the links on the /feed page are still shown as http (and if ProxyPreserveHost (apache), or other httpd equivalent, are not set, the links on the feed page show the proxied to address, e.g. http://localhost:5000/ ); fastmail's status page feed has this problem:
https://www.fastmailstatus.com/