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
I know what you mean... I just wonder that if someone is spoofing POST requests they may try to do something malicious with the payload, so we'd be better throwing it out before trying to process it at all.  This is just a vague worry though... I don't know for sure what could break things.