Change UI behavior based on `EmailAsUsername`

[panoukos41]

Problem

The register & login pages ignore the EmailAsUsername option.

Fix

• Update the RegisterInputModel validator to validate the username only when the option is disabled.
• Update the register page model to fill the username with the email if the option is enabled.
• Update the Register page by showing only email and password as a registration method when the option is enabled.
• Update the Login page to reflect the login method depending on the option.