layouts/*.html: use customer_footer.html to share analytics

[odeke-em]

This issue is a follow-up from PR #309. The tricky part is that the HTML pages in layouts/codelabs are generated by claat https://github.com/googlecodelabs/tools/tree/master/claat and hence are detached from the entire site. The HTML just happens to be manually copied into the site, hence sharing custom_footer.html is a hard task currently, unless am mistaken.

Let's think of a way to share it though.

/cc @hvent90