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: {F115902}
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T3886
Differential Revision: https://secure.phabricator.com/D8283