Add "user" and "users" standard custom fields

Summary: These end up a little weird with subclassing instead of switch, but some day we could alias them to one another or something I guess. If I'm feeling brave, I might get rid of the "user" variant when I migrate Maniphest custom field specs, and turn it into "users, limit = 1" or something like that.

Test Plan: See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7010