Allow "maniphest.subtypes" to configure which options are presented by "Create Subtask"
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.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D19854