Move subdomain logic to tenant model

We had some of the logic in the ApplicationMailer. Since we're going to
use this logic in more places, we're moving it to the Tenant model,
which is the model handling everything related to hosts.

spec/lib/tasks/sitemap_spec.rb

@@ -74,7 +74,7 @@ describe "rake sitemap:create", type: :system do
   end
 
   it "generates a sitemap for every tenant" do
-    allow(ActionMailer::Base).to receive(:default_url_options).and_return({ host: "consul.dev" })
+    allow(Tenant).to receive(:default_url_options).and_return({ host: "consul.dev" })
     FileUtils.rm_f(Dir.glob(Rails.root.join("public", "tenants", "*", "sitemap.xml")))
 
     create(:tenant, schema: "debates")