Create notifications for existing tickets

[dav3r]

🗣 Description

This PR causes notifications to be generated for existing tickets in certain specific situations, namely if an open (non-false positive) ticket or a re-opened ticket does one of the following:

• Its severity changes from less than 3 (High) to 3 or greater
• Its KEV flag changes from False to True

💭 Motivation and context

This is a requirement specified in https://github.com/cisagov/cyhy-system/issues/29.

Closing language:

• Closes https://github.com/cisagov/cyhy-system/issues/42

🧪 Testing

• [x] Verified that a notification was generated for an existing open (non-false positive) ticket that was severity 2, then changed to severity 3
• [x] Verified that a notification was generated for an existing open (non-false positive) ticket that had kev = false, then changed to kev = true
• [x] Verified that a notification was generated for a closed ticket that was severity 2, then re-opened as a severity 3
• [x] Verified that a notification was generated for a closed ticket ticket that had kev = false, then re-opened with kev = true

✅ 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.