Allow users to specify whether Maniphest custom fields should be copied or not when creating a "Similar Task"

Summary:
When you create a new task, the UI gives you the option to create another similar task. We copy some fields, but not others.

Currently, the field list is hard-coded and excludes auxiliary fields. Instead, allow auxiliary fields to elect to be copied.

Test Plan:

• Created a new task, verified appropriate field defuaults.
• Created a new "similar" task, verified 'copy' fields copied in.
• Edited an existing task, verified appropriate values.
• Edited-with-errors, verified new values didn't get reverted in the form.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1193

Differential Revision: https://secure.phabricator.com/D2410