Assert provided content is contained within an element found by ``id``.

def element_contains(self, element_id, value):
    """
    Assert provided content is contained within an element found by ``id``.
    """

    elements = ElementSelector(
        world.browser,
        str('id("{id}")[contains(., "{value}")]'.format(
            id=element_id, value=value)),
        filter_displayed=True,
    )
    if not elements:
        raise AssertionError("Expected element not found.")