Make system for creating lab bundles for a (set of) dashboard(s)

[vidartf]

This issue includes several parts:

• Have a config system to allow dashboard authors to declare lab-extension dependencies.
• Have a helper system for populating the above config value by capturing npm module dependencies from serialized widget state after a run.
• Have a system for creating a JS bundle (webpack?) for the dependencies from the lab dependencies from one (or more?) dashboards.
• Have a system for caching such bundles.

The motivation for this is to optimize JS downloads for dashboards, as the nbextension system does not have any de-duplication.

Related to #121 and #149.

[maartenbreddels]

During the dashboarding workshop, I started putting together a script to scrape some of this information. I should open a PR to share this code, it's very much a prototype btw.