R1:a6632f8c188a

Allow "maniphest.subtypes" to configure which options are presented by "Create Subtask"

Summary:
Ref T13222. Ref T12588. See PHI683. After D19853, "Create Subtask" may pop a dialog to let you choose between multiple forms.

Allow users to configure which forms are available by using `maniphest.subtypes` to choose available children for each subtype. Users may either specify particular subtypes or specific forms.

Test Plan: Configured "Quest" tasks to have "Objective" children, got appropriate prompt…