Prevent guest authors from logging in, disable account related functionality

A user with a role of Guest Author should not be able to log in at all.

* Password should be set to a long random password when a guest author is created
* Logging in should be denied at the authentication level
- Logging in at `wp-login.php`
- WordPress core application passwords
- REST API Basic Auth
- XML-RPC
* Ensure the guest author does not get sent an email when their account is created
* Disable password reset functionality for the user

[The Disable Accounts plugin](https://github.com/humanmade/disable-accounts) should do most of what we need. If there's anything missing, let's send fixes upstream to that plugin too.