[WIP] Lab app

[vidartf]

This is a work in progress on adding a jupyterlab plugin based way of adding dependencies and bundling them.

Noteworthy points:

• No method yet for a notebook to specify its dependencies (probably better in a separate issue). @jasongrout
• Currently adds lab shell to DOM as a hidden node, ideally we'd specify the shell ourselves to avoid putting any nodes with an id attribute on the page.
• Currently works with all examples that only use core widgets.
• There are some issues with duplication of dependencies due to lab/widgets using per-releases. Please do not consider merging before these are released in stable versions.
• Currently adds itself as a parallel bundle to the standard one, and overwriting some templates. This would need to be sorted out before merging, but allows for testing and comparison while working on it.

[jtpio]

Posting here for reference.

In the Voila weekly call yesterday we briefly discussed the possibility of using the refactored extension system in JupyterLab (planned for 3.0). So the lab bundles could be loaded in a similar way as with the classic notebook bundles (the way it is at the moment).