Make Viewer more extensible, allow for IPyWidgets support in new model

[quigleyj-mavenomics]

This isolates various parts of the viewer much more cleanly, and allows for better extensibility and maintainability.

Known issues:

• [x] Kernels are reused. We want fresh kernels each run.
• [x] ContentManager allows writes. We want this to be read-only (as much as makes sense)
• [x] Some CodeMirror extensions are throwing errors. The Viewer doesn't need those extensions, it should just exclude them.

To dos:

• [x] Add DocumentManager
• [x] Refactor NotebookRunner as a DocumentWidget
• [x] Run notebooks with new DocumentManager approach
• [x] Tie viewer toolbars to overall status bar
• [x] Implement IPyWidgets support
• [x] Implement Dashboard docs support (ideally just with a doc -> notebook translation).

Fixes #21 (for real this time)