Allow to resign from an accepted revision when you didn't accept the diff.

Summary: Girish wants to be able to do this.

Test Plan: Checked that I had the option in my sandbox on an accepted diff.

Reviewers: epriestley, jungejason

Reviewed By: jungejason

CC: aran, jungejason, tuomaspelkonen, epriestley

Differential Revision: 1020