Mark Owners package reviewers which own nothing in the current diff

Summary:
Ref PHI91. When Owners (or Herald, or manual user action) adds package reviewers to a revision, later updates to the revision make some of them less relevant or irrelevant.

Provide a hint when a package reviewer doesn't own any of the paths that a diff changes. Humans can then decide if the reviewer is obsolete/irrelevant or not.

This is a rough cut to get the feature working, design could probably use some tweaking if it sticks.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: jboning

Differential Revision: https://secure.phabricator.com/D18663