add redirect handler for notebooks by name

Now you can visit `localhost:8888/My Notebook.ipynb`,
and it will redirect you to the UUID url.
This allows cross-notebook links, etc. with `[other notebook](Notebook Name.ipynb)`
don't test for .ipynb in redirect handler

regex already guarantees it
Merge pull request #3058 from minrk/redirect

add redirect handler for notebooks by name