DisplayFieldElement.value2html() no longer converts a safe string to a ElementTree element.

Actor.detail_link no longer wraps the return value of as_paragraph() into a <div> tag. But DisplayFieldElement.value2html() and value2html() now wrap these values into <td> tags.

The general rule is that as_paragraph should return a safe string without any surrounding paragraph tags (like <p>, <li>, td>, <div>), this is done by the caller because it depends on the context.

Side effect: apply_cell_format() no longer works.