Move Abandon Revision action to the bottom if it's an admin action.

Test Plan:
Login as admin, look at an open revision you don't own, you should be able to
choose '(Admin) Abandon Revision', the option should be on the bottom and should
abandon the revision after sending the comment.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1060