Clarify OAuth identities

[fluffy-critter]

Technically an OAuth-based identity does not need a full user URL, it only requires providing the common domain name of the service (E.g. Mastodon.social, twitter.com, etc). It might be nice to signal that in some way, or to even make the standard UX just forward to the authority if the user clicks on the appropriate link for the service in the list (like linking to /login?me=http://twitter.com or whatever).

But it might be nice to just pretend that the full URL is important for the sake of consistency anyway. (But it might look like a bug if someone has two identities on a remote service and they want to specify one but the handler gives them another...)