Make the scheduler aware of KEVs

[felddy]

🗣 Description

This PR makes the scheduler aware of the KEV field in tickets. As per the requirements, when a host has a ticket with details.kev set to true the scheduler will schedule this with the same priority as a critical vulnerability.

💭 Motivation and context

These changes makes no sense. I have no idea why we are doing it. See: cisagov/cyhy-system#38 All vulnerabilities that end up in the "Known Exploited" catalog will already have high priorities from their CVSS scores.

Closing language:

• Placates and closes cisagov/cyhy-system#38
• Closes cisagov/cyhy-system#44
• Closes cisagov/cyhy-system#45

🧪 Testing

• [x] Tested by scanning a host with a vulnerability that is listed in the KEV but no other concurrent criticals on the same host.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [x] All new and existing tests pass.