During first-time setup, create an administrator account with no authentication instead of weird, detached authentication

Summary:
Ref T6703. Currently, when you create an account on a new install, we prompt you to select a password.

You can't actually use that password unless you set up a password provider, and that password can't be associated with a provider since a password provider won't exist yet.

Instead, just don't ask for a password: create an account with a username and an email address only. Setup guidance points you toward Auth.

If you lose the session, you can send yourself an email link (if email works yet) or bin/auth recover it. This isn't really much different than the pre-change behavior, since you can't use the password you set anyway until you configure password auth.

This also makes fixing T9512 more important, which I'll do in a followup. I also plan to add slightly better guideposts toward Auth.

Test Plan: Hit first-time setup, created an account.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: revi

Maniphest Tasks: T6703

Differential Revision: https://secure.phabricator.com/D20111