support `files` in 1.0

nbconvert in 1.0 strips the `files/` URLs, so the old redirect no longer works.

This adds a new restriction that notebooks must come from a URL ending in `.ipynb`, which we may not want. The alternative is to override the rm_files filter in nbconvert to do something else.

Are there (m)any notebooks currently served without their file extension? Do we want to support them?