Pin a variant as a suspect for a given case.

def suspects(case_id, variant_id):
    """Pin a variant as a suspect for a given case."""
    case_obj = app.db.case(case_id)
    variant_obj = app.db.variant(case_id, variant_id)
    app.db.add_suspect(case_obj, variant_obj)
    return redirect(request.referrer)