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