'Live' artifact view

[okennedy]

What pain point is this feature intended to address? Please describe.
In some circumstances, it is convenient to be able to see multiple artifacts simultaneously (e.g., plots, or dataframes). Yes, they're visible in the notebook, but it might be handy to 'pop' them out into a separate window.

Describe the solution you'd like

• Open a specific URL, see the artifact as of the end of the notebook
  • Or better still, as of a specific cell... or even have the ability to configure this.
• Make a change to the notebook and see the artifact live-updated.

Describe alternatives you've considered
This is a 90% version of #114, that should achieve most of the value, with a fraction of the effort.

Notes
I think this should be achievable through the existing branch-watcher socket. We could be a bit more efficient by computing dependencies at the server and only sending those... but let's get something working first.