Creating subtask in Maniphest also implies template from parent

Summary:
Refs T2598 (or should it fix)

If no template was specified, and we are creating a subtask of a task, we can safely assume that we
can take the parent as a template. Copy Projects, CCs and the like (even the guy assigned to).

Test Plan:
Only in my imagination.

Jk, I'm currently on Windows, and it always gives 404s (without chrome), so I can't test. I'll switch over to Ubuntu later today to test this.

I'm confident that this works, though.

Reviewers: epriestley

CC: hfcorriez, aran, Korvin

Maniphest Tasks: T2598

Differential Revision: https://secure.phabricator.com/D5499