Twenty Fifteen: improve naming and documentation of customizer js files to match core filenaming conventions.

Props celloexpressions, fixes #30160.


Built from https://develop.svn.wordpress.org/trunk@30271


git-svn-id: http://core.svn.wordpress.org/trunk@30271 1a063a9b-81f0-0310-95a4-ce76da25c4cd