Implement view and edit policies in Differential CustomFields

Summary:
Ref T3886. Ref T418.

• Adds "View Policy" and "Edit Policy" fields.
• Allows CustomFields to produce arbitrary types of transactions, so these fields can produce standard view/edit policy transactions and get all the strings and validation associated with them.

Test Plan: {F116001}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T418, T3886

Differential Revision: https://secure.phabricator.com/D8287