Provide a way for user to log in to Who Server with an email address. Who Server will send the user an email containing either a login link or an authentication code.
The user can enter their email address directly, or we can delegate to it via RelMeAuth or IndieAuth. In the former case, we should probably fall back to it if the identifier resembles an email address (e.g.
mailto:email@example.com). The Who library can probably detect a domain as being an email host but cannot resolve specific accounts on said host (which is probably a good thing).
If the user enters an explicit
mailto: URI, it's definitely an email address, although for certain Silo email services with authentication APIs, we can use those instead. This is the approach Portier takes.
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