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
Thinking about it... (and I know there are a lot of controllers that would need to change)... this check could be the first thing that happens in each 'save' method - before doing any validation or creating a page or anything - if page is submitted then these POST controllers should never be hit and should probably redirect right away without looking at the data.