Required billing fields and submitted form error fields support

[kimcoleman]

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Changes proposed in this Pull Request:

JS to support required billing fields and form field errors on submit.

Required fields indicated with an asterisk, screen reader text, and aria-required field attribute.

After form submit, fields that have errors are highlighted.

Other information:

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Changelog entry

• BUG FIX: Fixed styling for fields with errors on checkout form submit.
• BUG FIX: Fixed Billing Address fields required asterisk and aria-required attributed.