Add a "Create Subtask" operation to Maniphest

Summary:
Add a new "Create Subtask" action that allows you to quickly split a task into
dependent subtasks, using the parent task as a template.

Followups:

• Cause "workflow=<parent>" to change "Create Another Task" into "Create

Another Subtask" (after D736).

• Show parent tasks (requires some schema stuff).

Test Plan:

• Created a new task.
• Created a new subtask.

Reviewed By: codeblock
Reviewers: hunterbridges, codeblock, jungejason, tuomaspelkonen, aran
CC: aran, codeblock
Differential Revision: 774