Add state selection and regional admin option to form in Users.tsx

[ameliav]

🗣 Description

Add state selection and regional admin option to add new user form

💭 Motivation and context

Improves user experience. Allows a user to add a single regional admin from the user interface and requires a state to be selected per each new user.

🧪 Testing

New changes should be reflected and allow a new user to receive an email

📷 Screenshots (if appropriate)

Updated form

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced
  in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Create a release.

[nickviola]

LGTM. Tested and worked as expected locally.

[schmelz21]

lgtm