Propagate "edit policy" and "view policy" when creating a new template task

Summary:
Fixes T4158. Two possible refinements:

• Maybe we should make all of these things respect ManiphestCapabilityEditAssign::CAPABILITY, etc. I think it's reasonable either way, and this is probably more intuitive and useful for most cases.
• Maybe we should check that you can see the policies before copying them. Again, this is sort of reasonable either way.

Test Plan: Created a new task from a template, saw that it inherited policies.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T4158

Differential Revision: https://secure.phabricator.com/D7649