Reconsider putting fields in a <p> element

[sobjornstad]

While this seems a natural fit for the front or back of a card in most cases, it imposes some limitations on what content can go there. It may be better to switch over to a <div> element.

We'll need to preserve backwards compatibility in some way here; proving that we haven't messed up anybody's cards may be a challenge, so it might be better to save this for a major release.