Ensure that custom nodes are serialized/deserialized to/from html properly

[b-kelly]

Umbrella bug report to track general node serialize/deserialize issues. Serialize is important for copying nodes to the clipboard in a way that we can deserialize on a later paste event. Ideally, custom nodes should serialize as closely to their final rendered versions as possible so that the deserialize can handle both "copied from rich text mode" and "copied from a final rendered post".

Reported on Meta.SE in the following posts:

• https://meta.stackexchange.com/a/360079/395497