Improve UX debt and UI polish heatmaps

Improves the UX debt and UI polish heatmaps to:

• more clearly indicate that they contain all issues regardless of milestone
• have no severity/no priority table cells => #490 (closed)

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 =>
  • Week-in-review