Enable Phabricator admin to change the owner of a herald rule

Summary:
Added a typeahead in the edit herald rule page that allows an admin or
owner to change the current owner of a rule. If the typeahead is emptied, the
current owner will remain owner.

Test Plan:
Created a test rule. Changed the owner. Deleted the owner in the
typahead. Verified expected behavior.

Reviewers: jungejason, epriestley

Reviewed By: epriestley

CC: aran, jungejason, epriestley, xela

Differential Revision: https://secure.phabricator.com/D1322