Add a new policy to ask for severity to be set on UX-related issues

What does this MR do and why?

This is to ensure good hygiene of UX-related issues that should have a severity label set.

Expected impact & dry-runs

Labels	Number of issues pinged 2022-01-12	Number of issues pinged 2022-01-17	Number of issues pinged 2022-01-20
~"Actionable Insight"	76	68 (-8)	61 (-7)
Low SUS Score	45	45	45
UX scorecard-rec	11	6 (-5)	5 (-1)
cm-scorecard-rec	5	5	5
~"UX debt"	13	3 (-10)	2 (-1)
UI polish	37	28 (-9)	26 (-2)
UX solution validation	42	32 (-10)	32
accessibility	11	8 (-3)	8
VP-UX Dogfooding	0	0	0
UX + typebug	6	3 (-3)	5 (+2)

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 => @clenneville @vkarnes
  • Relevant Slack channels => #ux
  • Engineering week-in-review

Closes #851 (closed).