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/user_mailer.rb

@@ -4,6 +4,8 @@ class UserMailer < Devise::Mailer
   default from: ENV.fetch('SMTP_FROM_ADDRESS') { 'notifications@localhost' }
   layout 'mailer'
 
+  helper :instance
+
   def confirmation_instructions(user, token, _opts = {})
     @resource = user
     @token    = token