Customize: Implement customized state persistence with changesets.
Includes infrastructure developed in the Customize Snapshots feature plugin.
See https://make.wordpress.org/core/2016/10/12/customize-changesets-technical-design-decisions/
Props westonruter, valendesigns, utkarshpatel, stubgo, lgedeon, ocean90, ryankienstra, mihai2u, dlh, aaroncampbell, jonathanbardo, jorbin.
See #28721.
See #31089.
Fixes #30937.
Fixes #31517.
Fixes #30028.
Fixes #23225.
Fixes #34142.
Fixes #36485.
git-svn-id: https://develop.svn.wordpress.org/trunk@38810 602fd350-edb4-49c9-b593-d223f7449a82
83b059aa19