Fix for [#64347858] :  Don’t validate embeddables while duplicating them

It has to be assumed that the embeddables have already been validated in
the authoring system.

Note: I verified that I was able to edit an Xhtml item that wasn’t passing
validation, and then save that item using the WYSIWG editor, and it then
it started passing validations.

https://www.pivotaltracker.com/story/show/64347858