Break out copies into separate pages

This branch separates copies into individual pages at `/copy/123`.

There is now a form to jump directly to a copy from a start page, if you know the ID - for example, if you have found a copy of a book on the bookshelf and want to find it straight away.

I've also made `book_reference` (aka the copy number) the result of the `to_param` method, so paths now contain the `book_reference` rather than the `id`. Ideally, this would have been the Copy ID in the first place, but when I created the app, I didn't factor that in due to the existing numbering in place in the GDS Library. In the future, I will probably fix this.

There's also some more tests around borrowing books \o/