Ability to close poll

Summary: Fixes T3566 List of poll actions should include ability to close an open poll or reopen a closed poll.

Test Plan: Poll author should be able to close/reopen poll. Non-author should get policy screen when attempting to close/reopen poll.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T3566

Differential Revision: https://secure.phabricator.com/D8846