Refactor Threat Insights labels

What does this MR do and why?

Move the Threat Insights labels to a SSOT in lib/constants/labels.rb.

I did this because I wanted to reuse the same labels in !1687 (merged).

Expected impact & dry-runs

None. This is a refactor.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review