Create changeset prior to opening customizer for post preview

[westonruter]

There is currently somewhat of a hack when opening the customizer from the post edit screen. Changes from the edit post screen get stored in sessionStorage and then populate the customizer settings once it is loaded. This results in an unfortunate flash of unedited content in the customizer preview and it is no longer necessary now that we have changesets (in 4.7). When clicking Preview we can now create/update a changeset with the post/postmeta changes and then open the customizer with the changeset applied.