Re-evaluate policy when PR base branch changes

[bluekeyes]

If a PR's base branch changes, the policy-bot check context will have a different value and the policy may have different rules, but it looks like we don't respond to this webhook event. This means users must take some action (add a comment, push a commit, visit the details page) to post the updated status check.

While we do handle the edited action in the pull_request event, I have a feeling that trigger value is leading to skipped evalutation for some policies:

policy-bot/server/handler/pull_request.go

Lines 51 to 52 in 1d143b1

	case "edited":
	t = common.TriggerPullRequest

We probably need to always use a different trigger or change the trigger based on what was edited about the PR.