'Extract current field mode.
Returns:
`placeholder`/`edit`/`display`'

def mode_for_field(self, field_id):

    self.wait_for_field(field_id)
    query = self.q(css='.u-field-{}'.format(field_id))
    if (not query.present):
        return None
    field_classes = query.attrs('class')[0].split()
    if ('mode-placeholder' in field_classes):
        return 'placeholder'
    if ('mode-display' in field_classes):
        return 'display'
    if ('mode-edit' in field_classes):
        return 'edit'
