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