refactor: refactor DNS resolution

This commit also adds some of tests for the Domain model. It was during the writing of these tests that the DNS resolution refactoring requirement became apparent.

app/controllers/domains_controller.rb

@@ -71,7 +71,7 @@ class DomainsController < ApplicationController
     when "Email"
       if params[:code]
         if @domain.verification_token == params[:code].to_s.strip
-          @domain.verify
+          @domain.mark_as_verified
           redirect_to_with_json [:setup, organization, @server, @domain], notice: "#{@domain.name} has been verified successfully. You now need to configure your DNS records."
         else
           respond_to do |wants|