Use cookie for anonymous user ID instead of localStorage
Created by: felixfbecker
Stores the anonymous user ID in a cookie instead of in localStorage so it can be read by subdomains (about.sourcegraph.com).
I looked into the new async cookiestore API but it is not supported yet by any other browser than Chrome and still a draft spec. Used a library though because I don't want us to deal with cookie strings.
Closes #11658 (closed)