Fix a bug with "Resign" in Diffusion

Summary: The PHID list is a list, not a map -- I must have broken this in refactoring or something, since everything else works fine. See D2013.

Test Plan: Viewed a resignable revision, saw "Resign" (new after this commit), resignd.

Reviewers: 20after4, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D2417