Define ajaxurl in the customizer.

props celloexpressions.
see #25866.
Built from https://develop.svn.wordpress.org/trunk@26082


git-svn-id: http://core.svn.wordpress.org/trunk@26002 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove the code for setting ajaxurl from Twenty Fourteen.

props celloexpressions.
fixes #25866.
Built from https://develop.svn.wordpress.org/trunk@26083


git-svn-id: http://core.svn.wordpress.org/trunk@26003 1a063a9b-81f0-0310-95a4-ce76da25c4cd