Restored Skip to Main Content functionality (CRASM-630)

[hawkishpolicy]

• Restored "Skip to Main Content" functionality.

🗣 Description

• Added the "main-content" id to the Box div inside of Main.
• Set the tabIndex to -1 for the Box div inside of Main.

💭 Motivation and context

• The refactor of Layout following the creation of the new Side Drawer removed the functionality of the "Skip to Main Content" component.
• The restoration of the "main-content" id and -1 tabIndex restores this functionality.
• Closes CRASM-630
• Closes Skip To Main Content Not Working (CRASM-630) #589

🧪 Testing

📷 Screenshots (if appropriate)

✅ 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.

[chrtorres]

LGTM

[nickviola]

LGTM

[cduhn17]

LGTM