migrate away from /notebooks/<notebook-id> to /<username>/<notebook-id>/

Going to create a new (explicit) issue for this to keep track of progress.

This will help clarify a number of issues regarding routes:

- a user page will simply be at `/`
- the most recent version of a notebook will be at `//`
- any diagnostics, additional info, etc. can be at `///`, so for instance `/hamilton/425/revisions/` would give you a page to view all the revisions of a notebook. Perhaps `activity` is a better word here than `revisions` because that will also open the door for comments, observing the fork tree, etc, but that can be approached separately.
- we can then view a particular revision of a notebook with `///revisions/` or something to that effect. This somewhat follows how Github handles gist routing.

@wlach @bcolloran would love your thoughts on this.