feed: add published advisory atom feed
feed: add published advisory atom feed

Implement /advisory/feed.atom which provides an atom feed for
published advisories.

anthraxx/arch-security-tracker,archlinux/arch-security-tracker,jelly/arch-security-tracker,jelly/arch-security-tracker,anthraxx/arch-security-tracker,archlinux/arch-security-tracker,anthraxx/arch-security-tracker