Define federation end-points for managing one's photos

If a domain exposes support for Libravatar, then its users won't be using www.libravatar.org to upload photos and associate them with email addresses.

So if a third-party site wants to direct its users to the site where they need to upload their photo, they should have a way to generate a link to the right place.

Essentially, they need a link like: avatars.example.com/avatar-manage/{email_address} (so it would use the _avatars._tcp.example.com SRV record or the _avatarssec._tcp one if the first one is missing).

Furthermore, domains should be able to expose an embeddable flow too, as discussed in bug 1117643.