Fix Differential transaction strengths

Summary: Fixes T4899. Action strengths got lost somewhere along the way; actions like "Accepted" should be stronger than "Changed Subscribers".

Test Plan: Verified things sort as expected now, with major actions at the top.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4899

Differential Revision: https://secure.phabricator.com/D8857