Support indieauth-metadata endpoint

[fluffy-critter]

IndieAuth now supports a bundled metadata discovery mechanism, and it would be a good idea for Authl to support that for server discovery as well.

indieauth.find_endpoints needs to check for the indieauth-metadata rel, and if it exists, consume the data from there (overriding the data discovered from the original resource).

The endpoints of interest to Authl and its primary consumers are:

• authorization_endpoint (for IndieAuth's login flow)
• ticket_endpoint (for TicketAuth support, primarily in Publ)

Additionally, if there is now an issuer, that needs to be added to the token state and verified in IndieAuth.check_callback.