Use slugs to identify notebook, instead of numeric ids

@jaredkerim suggested using slugs to identify notebooks on the server instead of numeric ids. So e.g.

`https://iodide-server.iodide.io/notebook/what-does-an-iodide-notebook-look-like/`

instead of (what we currently have):

`https://iodide-server.iodide.io/notebook/1/`

This (1) makes it easier to identify what a url points to and (2) frees us up to make database changes without breaking users.