Support RelMeAuth

[fluffy-critter]

A lot of folks (and IndieWeb guidance) still refer to RelMeAuth. This could be supported in Authl, although it might not be particularly straightforward. Idea for the flow:

1. RelMeAuth provider has access to the Authl container
2. For each rel="me" link on the identity page, see if Authl supports it, and if so, use that
3. Proxy the identity flow for the returned identity link
4. Return the original URL as the profile page. Bonus: Fuse the identity provider's profile with the h-card profile?

OPTIONAL: It would be nice if Handler.handles_page is able to provide an extended name, e.g. "RelMeAuth (via Twitter)". Since handles_page currently returns bool it could be changed to return Union[bool,str] and if it's a str that's a signal that it overrides the name.

[fluffy-critter]

#108 does a partial implementation of RelMeAuth, but for it to be complete, it needs to check the backlink to the profile page and then report that as the canonical profile URL, which is also much more involved. Coming up for an appropriate flow for true RelMeAuth that works within the Authl API is proving difficult.

Perhaps treating RelMeAuth as a fully-fledged provider and having it implement the callback flow as a proxy thing with the underlying provider onion-wrapped inside would work, but I start to worry about complexity and attack surface.