Add gitlab.io to the Public Suffix List

As users of GitLab can be granted arbitrary domain names under gitlab.io using GitLab Pages, *.gitlab.io should appear in the Public Suffix List.

https://publicsuffix.org/submit/

This will allow users of the PSL (which includes browsers and Lets Encrypt, among others) to treat each subdomain as an independent entity, rather than as a set of related entities. This has positive implications for, e.g., Lets Encrypt rate-limiting.