Support native Persona efforts

[callahad]

Goal

Ensure that we're not missing anything critical to native support in user agents.

Define

• [x] Review / discuss native implementation requirements (include ozten and jedp)
• [x] Document the authority discovery process (don't forget the domain hint when following delegation!)

  Implement

• [x] Make the fallback a true IdP

  Validate

• [x] Prototype native integration.
  • [x] Successfully log in with a native identity provider.
  • [x] Successfully log in with the fallback.

    Finish

• [ ] Blog about our experiment.