Add route GET /contents/<book-ident-hash>:<page-ident-hash>

Redirect to /contents/<page-ident-hash> if it's in the book, otherwise
404

Close #269 

https://trello.com/c/b9S7kZaE/973-change-module-numbering-in-collections-to-be-the-uuid-of-the-module-instead-of-the-placement-within-the-collection