Refactor the Customizer accordion so that it can be used in other locations.

fixes #23449. props lessbloat, aaroncampbell

git-svn-id: https://core.svn.wordpress.org/trunk@23417 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Compress scripts/styles: 3.6-alpha-23417.

git-svn-id: https://core.svn.wordpress.org/trunk@23418 1a063a9b-81f0-0310-95a4-ce76da25c4cd