Fix bug where the user was unable to shift navigate from 'Assigned To' to 'Title' field in Maniphest

Test Plan: go to the new task page, go to the 'Assign To' field, assign the task to someone and try to do a Shift-Tab to return to the title field

Reviewers: igoleo, btrahan

Reviewed By: btrahan

CC: aran, Korvin

Maniphest Tasks: T2760

Differential Revision: https://secure.phabricator.com/D5547