Set autocomplete attr for email field on signup page (#19833)

The email address will be used as the "username" for sign-in purposes, so it's the value that should be stored in password managers. We can inform the password manager of this by setting `autocomplete="email"`. Without this hint, password managers may instead store the `username` field, which isn't valid for sign-in (this happens with iCloud Keychain in Safari, for instance).