Added Severity Label validation to All Vulns table (CRASM-822)

[hawkishpolicy]

• Extends severity level validation used in VulnerabilityBarChart.tsx to the All Vulnerabilities table.
• This ensures continuity of Severity labels between the bar graph and table.
• Also ensures that server side based filtering returns accurate results.

🗣 Description

• Ensured that null or empty string values are displayed as 'N/A' in the severity column of the All Vulns table.
• Ensured that values of Other or any value that does not include 'Critical', 'High', 'Medium', or 'Low' are displayed as 'Other' in the severity column of the All Vulns table..
• Formatted the severity column of the All Vulns table to display the severity label in title case.
• Updated the vulnerability search endpoint to return vulnerabilities with a severity of 'Other' if the severity is not 'Critical', 'High', 'Medium', or 'Low'.
• Updated the vulnerability search endpoint to return vulnerabilities with a severity of 'N/A' if the severity is null or an empty string.
• Updated the vulnerability search endpoint to return vulnerabilities regardless of the case of the severity value.

💭 Motivation and context

• Closes CRASM-822

🧪 Testing

• Seeded local database with numerous vulnerabilities containing irregular severity labels.
• Filtered via table and bar graph to see if accurate results were returned.

📷 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.

[lwersiy]

Looks good

[jayjaybunce]

Found two small bugs, resolved these together with @hawkishpolicy

LGTM after checks pass

[chrtorres]

LGTM

[schmelz21]

LGTM