Support "template objects" generically in EditEngine

Summary:
Ref T9132. Ref T9908. Fixes T5622. This allows you to copy some fields (projects, subscribers, custom fields, some per-application) from another object when creating a new object by passing the ?template=xyz parameter.

Extend "copy" support to work with all custom fields.

Test Plan:

• Created new pastes, packages, tasks using ?template=...
• Viewed new template docs page.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5622, T9132, T9908

Differential Revision: https://secure.phabricator.com/D14699