Infer priority label from severity label for bugs

Summary

Infer priority label from severity label for bugs

- If severity 1 or 2 exists set appropriate priority label
- If priority label already set do not do anything
- Set no default priority for severity 3 and 4

References

Related to #400 (closed), #525 (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