Restrict marking task duplicate to "merge duplicates" workflow

Summary: Fixes T4819, remove status "duplicate" from dropdown in edit task unless task is already in duplicate status

Test Plan: Edit task, not in duplicate status, verify dropdown does not have "duplicate" option. Edit task already in "duplicate" status, verify that dropdown shows "duplicate" status option.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T4819

Differential Revision: https://secure.phabricator.com/D8902