Provide visual view of stats

[rgbkrk]

This is a dupe of jupyter/tmpnb-redirector#1, but likely belongs in both tmpnb and tmpnb-redirector since they're using the same "spec".

I'm kind of wishing we had a beautiful front-end stats page (which could just be at stats.example.org), or we could make two endpoints on a given tmpnb server /stats for a beautiful mockup of the data and /api/stats for the raw JSON.

In addition, some things I've wanted during a deploy:

• List version of the containers that are running
• Some aggregated activity data (e.g. number of docker logs of individual containers)

[rgbkrk]

We should try to adhere close to the same spec as the binder project for these temporary environments. This one is a bit tricky for operations since currently tmpnb-redirector relies on the /stats endpoint for JSON.

We'll just have to do a migration once we conform to an API + endpoints.

[minrk]

yeah, keeping JSON under /api/ seems to be a consistently good idea. A human version of that rendered on /stats seems sensible enough.

[rgbkrk]

Done in #162