Super Properties and subdomains
http://api.mixpanel.com/site_media/js/api/mixpanel.js) that will ensure that
the Super Properties cookie is valid across all subdomains of your website.
Previously, different subdomains would have their own super properties cookie,
and values you set on one domain wouldn’t be included in the others.
You still have the option to track subdomains separately though; take a look
at mpmetrics.set_config() in the documentation here: