Reevaluate PRs when policy.yml file is changed

[jmcampanini]

When the .policy.yml file is updated, existing PRs need some kind of action (comment, reopen, etc.) to trigger reevaluation with the new policy. We should either:

1. Listen to push events, watch for policy.yml changes on the default or target branch, and reevaluate open PRs
2. Provide an API endpoint someone can POST to that will reevaluate all open PRs on a repository

[NargiT]

This would be great especially with remote policy files