function retainmentAllowed(attachmentIds, shapes) {
  var allowed = 'attach';
  forEach(shapes, function(shape) {
    if (attachmentIds.indexOf(shape.id) === -1) {
      allowed = false;
      return;
    }
  });
  return allowed;
}
Returns 'attach' if all shape ids are contained in the attachmentIds array.
Returns false if at least one of the shapes is not contained.

@param {Array<string>} attachmentIds
@param {Array<Object>} shapes