Use policy generator for slipping issue check

What does this MR do and why?

Describe in detail what your merge request does and why.

A small MVC for: #839 (closed) to use a policy generator, as mentioned in !1019 (diffs, comment 756172831), which let's us pass some dynamic data to policies if needed.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

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 => @oregand
  • Relevant Slack channels => g_create_editor
  • [-] Engineering week-in-review