Formatted Vuln Details Severity Level (CRASM-1054)

[hawkishpolicy]

• Brought the formatting of irregular severity levels displayed inside of Vulnerability Details into line with the remainder of the app.

🗣 Description

• Title case is now applied to all vuln severity levels.
• Null, undefined, N/A, and empty strings are now formatted to 'N/A'.
• Low, Medium, High, and Critical are now formatted to 'Low', 'Medium', 'High', and 'Critical' respectively.
• All other values are formatted to 'Other'.

💭 Motivation and context

• Ensures the formatting of irregular severity levels remains constant across the app.
• Closes CRASM-1054

🧪 Testing

• tested locally

📷 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 relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All new and existing tests pass.

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Create a release.

[nickviola]

LGTM