Ch 819: editHtml personalize element variation

Allow a variation type to be retired by still available for backwards compatibility.
Add an editHtml personalize variation type.
Created a callback mechanism for edit in context:
* I placed this in the main personalize_elements.js file and I'm conflicted about that.  The structure was there to support multiple callback types per variation type but it's technically the admin layer and not the main layer.  However, I also don't want to add even more javascript files to the page when we only need a little callback.  I welcome new thoughts on this... it would be easy enough to move if there was something else that worked better and felt less cludgy.
What a shame to have to reference any of this here :(