Add support for friends' diary entries RSS feeds

[spughettisauce]

URL

https://www.openstreetmap.org/diary/friends
https://www.openstreetmap.org/diary/friends/rss

RSS feeds currently aren't supported for friends' diary entries

[tomhughes]

Because it would be of very limited use - it would require you to be logged in in order to know who your friends were and most feed readers are not going to be able to do that.

[mmd-osm]

Some sites generate URLs with personal token https://gitlab.com/group/project.atom?feed_token=XXX
This should probably work even when a user can't enter any credentials. I've seen this frequently for iCal subscriptions as well.