Modify Header.tsx to include conditional nav items for regionalAdmin User Type

[nickviola]

Modify Header.tsx to include conditional nav items for regionalAdmin User Type

🗣 Description

Closes #22 - Adds "User Registration" Nav item for regionalAdmin users which navigates to "/region-admin-dashboard" path.

💭 Motivation and context

Adding Nav item to manage users as a regionalAdmin userType

🧪 Testing

As a "regionalAdmin" userType, you should see the "User Registration" menu nav item in the top right hand dropdown. It should not be there for any other user types.

✅ 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.

[ameliav]

LGTM!