Fix UI Header Response to Browser and Crossfeed Logo Routing

[chrtorres]

🗣 Description

• Fixed an issue preventing responsiveness from the UI header.
• Adjusted NavLink styling.
• Corrected Crossfeed logo routing incorrectly.

💭 Motivation and context

Closes issues 16 & 17.

🧪 Testing

Sign in as different user types and see if the header is correctly displayed and responsive.

📷 Screenshots

Full screen design

Small screen design

✅ 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.

[cduhn17]

LGTM

[nickviola]

LGTM

[nickviola]

Tested in both Safari and Chrome.

Chrome:

Safari:

All tested and works as expected.