Customizer: Add stable sorting for panels, sections and controls in JS. Improve sorting in PHP.

props westonruter.
fixes #30225.

git-svn-id: https://develop.svn.wordpress.org/trunk@30214 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: Make sure that the edit link will always be right of it's icon.

Props iamtakashi, fixes #30171.




git-svn-id: https://develop.svn.wordpress.org/trunk@30215 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: Use new core navigation tags.

Props obenland, iamtakashi, fixes #30189




git-svn-id: https://develop.svn.wordpress.org/trunk@30216 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: kern everything -- it reads better.

Props afercia, fixes #30195.




git-svn-id: https://develop.svn.wordpress.org/trunk@30217 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: escaping for the HTML5 shiv call.

Props philiparthurmoore, fixes #30212




git-svn-id: https://develop.svn.wordpress.org/trunk@30218 602fd350-edb4-49c9-b593-d223f7449a82