Implement new-style "Summary" field

Summary:
Ref T3886.

• Adds "Summary" field.
• Adds "CoreField" for fields stored on the actual object, to reduce code duplication a bit for the main fields.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3886

Differential Revision: https://secure.phabricator.com/D8283