Replace Differential "unit stars" with icons

Summary:
Ref T9764. These "star" icons are unclear, inconsistent, and not friendly to colorblind users.

They date from a time long ago when the product didn't have icons.

Modernize them and make them more consistent with the similar statuses in Harbormaster.

Test Plan:

Maniphest Tasks: T9764

Differential Revision: https://secure.phabricator.com/D21639