Hide 'newUser' text when registration form is hidden

[zattak1]

Summary

• When the multiple-account discouragement feature hides the registration form fields, the "or create a new account below" text remained visible, creating a dead-end message referencing content that doesn't exist
• Added a Users_login_newUser class to the newUser div so it can be targeted
• Included .Users_login_newUser in the selector that hides form elements when localStorage has the register key and dontDiscourageMultipleAccounts is not set

Test plan

• Enter an email that was previously used to register (triggers emailExists flow)
• Verify the "or create a new account below" text is hidden along with the registration form fields
• Verify the "Did you try to register..." and resend button still appear correctly
• With dontDiscourageMultipleAccounts option set, verify both the text and form fields appear normally

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