Canonicize user identity URLs

[fluffy-critter]

…uth requests

Summary

Canonicizes identity URLs to make the netloc case-insensitive and path non-empty.

Supports rel="canonical" on TicketAuth grant requests. Fixes #486

Detailed description

The users http://example.com, http://Example.com, and http://example.com/ should all be equivalent.

Also, since TicketAuth has no mechanism for the endpoint to provide its own canonical identity, the only way for TicketAuth to prove ownership of a canonical URL is for the profile to provide rel="canonical" and for the ticket granter to forward its request to that canonical URL. So this change will forward a TicketAuth grant request if rel="canonical" is provided, with no trust given to the forwarding page (to avoid the situation where for example https://alice.example provides a TicketAuth endpoint and also a rel="canonical" of https://bob.example).

Developer/user impact

Test plan

Added appropriate unit tests.

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