do not run bushido subdomain validation if the value does not get changed + prevent to change the subdomain in Heroku (non multi-sites mode)

app/views/admin/current_sites/_form.html.haml

@@ -15,7 +15,7 @@
     = f.custom_input :subdomain, :css => 'path' do
       %em
         http://
-      = f.text_field :subdomain
+      = f.text_field :subdomain, :disabled => (manage_subdomain? ? '' : 'disabled')
       \.
       %em
         = application_domain

lib/locomotive/hosting/bushido/custom_domain.rb

@@ -22,7 +22,7 @@ module Locomotive
           protected
 
           def subdomain_availability
-            return true if self.new_record?
+            return true if self.new_record? || !self.subdomain_changed?
 
             unless ::Bushido::App.subdomain_available?(self.subdomain)
               self.errors.add(:subdomain, :exclusion)