Limit number of EditEngine tokenizer tokens in "Owner" field UI to 1

Summary:
Ref T9132. Only allow a task to have a single owner in the UI.

In Conduit, make this field appear and behave as "phid" instead of "list<phid>".

Test Plan: Edited a task with new fancy form, got limited to one owner. Assigned/unassigned. Used Conduit to assign/unassign.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14666