Fix service adding to an existing doc in admin

- Allow name edit on new inline services of a doc (without that, it's not
decently possible to add a service from the document view).

costs/admin.py

@@ -25,7 +25,7 @@ class ServiceInline(admin.TabularInline):
         'name', 'subscriptions_count',
         'monthly_unit_cost', 'new_subscriber_cost')
     readonly_fields = (
-        'name', 'subscriptions_count',
+        'subscriptions_count',
         'monthly_unit_cost', 'new_subscriber_cost')
     show_change_link = True
     extra = 0