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
