Design auto-draft REST API

For Visual Editing PSP2, the content REST API should be extended to support the "auto-draft" feature. When the front-end requests auto-draft, a document's draft node should be updated (writeTo), but the changes shall not be "committed" to the preview node.

Next to the auto-draft call, we still need to support the old "save" call, which also commits the changes, or we could consider separating the saving from the committing at the API level.