Unit assignment - selection to relevant department only

[abandoned-prototype]

When in the "Edit Officer Assignment" form (https://openoversight.com/officer/<officer_id>/assignment/<assignment_id>) the Unit dropdown shows all units in the database, not just those of the officer's department. Only the units that belong to the officer's department should be shown here and the backend part of the form should also reject trying to assign a unit from a different department.