Adds a heat map for stuck issues

From Draft to Ready

• Add specs for the new heatmap
• Do a few dry-runs to see the new heatmap in action

Context

Closes #1404 (closed)

What does this MR do and why?

Adds a heat map for "stuck issues", and adds this heatmap for missed SLO bugs only.

This new heatmap is not based on priority/severity, but on severity, and some other criteria (e.g. no assignees, no milestone, Milestone: Backlog, Milestone in the past).

Why do we change so much in the code for just a new heatmap?

The existing heatmap logic was heavily tied to the concept of priority/severity as rows/columns.

This MR refactors most of the logic there to have other rows.

Expected impact & dry-runs

Dry-run

export TRIAGE_POLICY_FILE=policies/generated/team-summary.yml.erb/import_and_integrate.yml

https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/5531109718

Generated the following two tables:

Heatmap for SLOMissed bugs

	severity1	severity2	severity3	severity4	No severity
priority1	0	0	0	0	0
priority2	0	3	4	1	0
priority3	0	1	29	6	0
priority4	0	0	8	15	0
No priority	0	0	37	34	0

---

Heatmap for SLOMissed bugs (stuck issues)

	severity1	severity2	severity3	severity4	No severity
No assignees	0	3	74	53	0
No milestone	0	0	9	11	0
Milestone: Backlog	0	4	61	42	0
Milestone in the past	0	0	2	0	0

---

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (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