Primary domain shouldn't be a URL

[Dave Shoreman]

Validation in SiteController defines primary_domain to be a URL, but really we want a domain/FQDN. Laravel doesn't support that by default, so it's gonna need a custom rule.

Example of a custom rule here: https://stackoverflow.com/questions/47122338/laravel-eloquent-rule-for-domain-name

The primary_domain should be a plain FQDN because it'll be used for various things including Nginx configs where it won't have the https:// or any other protocol/path with it.