Investigate replacing PublicSuffix with Rails' builtin TLD handling

[aburgel]

We use PublicSuffix to extract the domain of the host so we can pull out the correct wildcard domain.

An alternative may be to use config.action_dispatch.tld_length to do the same thing, since we should be able to rely on the app to have this configured if it is running on a domain with a 2+ level TLD.

[jmileham]

One advantage of the dynamic approach we have is that if you're, e.g. operating a multi-tenant app with multiple host names, some of which have multi-level public suffixes, and you have TT wildcard cookies enabled, this would Just Work for you. It may be that with the PR you just merged the existing pain points of PublicSuffix domain validation have been addressed.