Allow regular staff members to show assigned open tickets on open queue

Previously only admins and department managers could show assigned tickets
on open queue -even- when the feature is disabled globally. This pull
request extends the same previlege to all agents.
Merge pull request #751 from protich/issue/show-assigned-tickets

Allow regular staff members to show assigned open tickets on open queue

Reviewed-By: Jared Hancock <jared@osticket.com>