Instance helper to replace site title helper (#2038)

* Move site title helper to instance helper (name change only)

* Remove newline in <title> tag

* Add site_hostname helper method to wrap up local_domain value

* Use site_hostname helper in places that need local_domain value

app/mailers/application_mailer.rb

@@ -3,4 +3,5 @@
 class ApplicationMailer < ActionMailer::Base
   default from: ENV.fetch('SMTP_FROM_ADDRESS') { 'notifications@localhost' }
   layout 'mailer'
+  helper :instance
 end