Add EntityEmbedEntityManager.

This helps reduce the amount of code we're having to use for entity loading, while also remaining testable. We could easily be using this on the CKEditor integration branch too.
The reasoning is fine with me, but there's something else that's bugging me. Once in WYSIWYG we determine whether the specified ID is a valid UUID or not and fill in 'data-entity-id' or 'data-entity-uuid' accordingly. Then again here in the load function we are checking whether we have a valid UUID or not. Can we do something to reduce this to just once check rather than doing the same check twice?