Add KEV flag to ticket documents

[felddy]

🗣 Description

Add the new KEV flag to the details section of ticket documents.
Generate notification documents for KEV flags as we do for high and criticals.

💭 Motivation and context

Resolves cisagov/cyhy-system#41 and resolves cisagov/cyhy-system#43 Note: Sorted a bunch of thangs. You can probably just focus on reviewing the non-sorting commits.

🧪 Testing

• [x] Tested in development cyhy environments by adding a host that had a CVE listed in the KEVs. Notification documents were generated as expected.

✅ 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.