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 behavior. Used "subtypes" and "forms" to select forms; used "forms" to reorder forms.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222, T12588

Differential Revision: https://secure.phabricator.com/D19854