R1:758db327d371

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…