support 1.0-style `files/` relative URLs

via redirect

only redirect on would-be 404
test 'files/' redirects

1.0 notebooks with links should work in 2.0
use request.text for Python 3 (content is always bytes)
Merge pull request #4527 from minrk/files

Add redirect for 1.0-style 'files/' prefix links, this makes it easier to develop notebooks on master and share them with 1.x users.