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

[hamilton]

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 /<username>
• the most recent version of a notebook will be at /<username>/<notebook-id>
• any diagnostics, additional info, etc. can be at /<username>/<notebook-id>/<some-page>, 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 /<username>/<notebook-id>/revisions/<revision-id> or something to that effect. This somewhat follows how Github handles gist routing.

@wlach @bcolloran would love your thoughts on this.

[wlach]

The one thing that's a bit confusing about this is that notebook ids really have nothing to do with the user per-se.

I was actually hoping we could switch to using slugs for identifying the notebooks (#741) and then implementing something like this. Might require a bit of thought/work to handle the edge cases there though (e.g. renames)