Remove frontend code for leaf pages

Leaf pages are no longer served by this app. Remove the code and redirect links as appropriate.
This won't work.  `private-frontend` is an instance of [frontend](https://github.com/alphagov/frontend) that has access to draft content.  It therefore only works for content that's handled by the frontend application.  We don't yet have a means of previewing content that uses the content-store.  

Having said that, given this app has no workflow at the moment, how did it do preview without also saving the item (and therefore publishing it)?