Loop subtask creation instead of task creation when in a subtask workflow

Summary: Simplify the division of a parent task into several subtasks by looping
the "create subtask" workflow. This replaces "Create Another Task" with "Create
Another Subtask" when you arrive via subtasking.
Test Plan:

• Created a task, looped task create flow.
• Created a subtask, looped subtask create flow.

Reviewed By: codeblock
Reviewers: hunterbridges, codeblock, jungejason, tuomaspelkonen, aran
CC: aran, codeblock, epriestley
Differential Revision: 779