Ensure that completed services can't be edited

Once a listing has been marked as completed it should not be editable without returning it to drafts.
- The "edit" links won't appear on the summary page
- If the user hacks the URL to get to an individual page this page will return "not found"
- If the user still tries to make a `POST` request to save the page nothing will be saved and the user will be redirected to the summary
Should there be a flash message to let them know that changes haven't been made to the listing?  Or is this really only going to happen if they have spoofed a POST request, in which case no messaging necessary because they should know better.