Add heatmap for customer bugs to group triage report

Summary

Adds a new section to each group triage report to represent the customer labelled bugs for that group. This provides a breakdown of the open bugs by their assigned severity and priority labels

Example:

Generated example (for 29th Jan 2021) for ~"group::access" customer ~bug labelled issues

Heatmap for customer bugs

	severity1	severity2	severity3	severity4	No severity
priority1	2	0	0	0	1
priority2	1	11	2	0	0
priority3	0	2	26	1	3
priority4	0	0	20	9	0
No priority	0	0	16	9	53

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations => gitlab-com/www-gitlab-com!73323 (closed)
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Week-in-review

Relates to #185 (closed)