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