Allow Maniphest task status to be changed on the "Edit Task" interface

Summary: We omit a 'status' dropdown when creating a new task since it's silly
to create a non-open task and would just clutter the interface, but there's no
reason not to allow status to be edited on the "Edit" interface.
Test Plan: Created a new task (no status dropdown). Edited a task, including
changing the task status.
Reviewed By: jungejason
Reviewers: toulouse, jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 720