While pushing `contentMetadata` to the DAO in free form like this could be unsafe due to having a vector to update `etherpadHtml` directly, this is fine since only global administrators can invoke this method. I haven't had a chance to verify this already exists, but I would like to have a unit test where a content manager tries to use setPreviewItems to set the etherpadHtml using the `contentMetadata` parameter, and verify it fails -- with some good documentation explaining why that test exists.

On a partially related note, the `etherpadHtml` should be moving off of the content object profile and onto the revision object, right? If that is done as part of this PR then I don't think the above unit test is necessary.