Implement "user" and "users" Maniphest custom fields

Summary:
Ref T2575. Implements "user" (zero or one users) and "users" (zero or more users) field types.

Also allows custom fields to participate in the handle pipeline.

Test Plan: {F35071}

Reviewers: hach-que, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2575

Differential Revision: https://secure.phabricator.com/D5287