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.
OK then! Let's leave it the way you have it :)