emails: hardcode nondisposable email list.

Hardcode the list of nondisposable email providers instead of making it
a config option. Also add a few new providers.

This was previously a config option to keep it secret, but there's not
much need for secrecy here.

A Restricted user's email must be on this list to unrestrict their
account. If a user is Restricted and their email is not in this list,
then it's assumed to be disposable and can't be used to unrestrict their
account even if they verify their email address.

app/models/email_address.rb

@@ -32,7 +32,7 @@ class EmailAddress < ApplicationRecord
   end
 
   def self.restricted(restricted = true)
-    domains = Danbooru.config.email_domain_verification_list
+    domains = EmailValidator::NONDISPOSABLE_DOMAINS
     domain_regex = domains.map { |domain| Regexp.escape(domain) }.join("|")
 
     if restricted.to_s.truthy?