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.
So I like it as a filter because 1) it allows any existing calls to the hook from outside this module to remain the same since there was no concept of active/inactive before; and 2) it matches the already established style of using a filter by permissions - although I could see this argued based on the fact that permissions filtering is more intelligent.  I do actually rely on grabbing them all and filtering myself in personalize_elements.admin.inc 120 in order to allow the full list of active options + the current option to be included in the options for editing an existing element.  Without the active property I guess I could just retrieve them twice instead (once active and once inactive if the current type is not found) in order to achieve the same result.  It seems that this gives me a bit more flexibility to wrangle things (but perhaps redundant if I can't think of any other case for this).