Add "reopens" prefix

Summary: Ref T1751. Add a "reopens" prefix for the "open" status. This allows commits to reopen tasks.

Test Plan: Pushed a commit containing Reopens Tx and saw Tx reopened.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T1751

Differential Revision: https://secure.phabricator.com/D12652