Implement a "Reviewers" CustomField
Summary:
Ref T3886:
- Adds a "Reviewers" field as a modern CustomField.
Ref T418:
- Allows CustomFields to emit transaction metadata.
Test Plan: {F116254}
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T418, T3886
Differential Revision: https://secure.phabricator.com/D8291