Save widget state in notebook save

[blois]

Currently when ipywidgets are used and the notebook is saved then re-opened the widgets are not rendered on open.

There are a number of hidden complexities here:

• We do have the widget state in the notebook model and can serialize it into the .ipynb file, but on load we need to figure out how to disambiguate widget model state carried over from the previous execution session and widget model state from the current kernel.
• The widget binary that we're currently rendering has a very leaky public API which is difficult to guarantee backwards compatibility. Currently we're making it fairly obvious by not persisting at all. This has a lot of overlap with Support installation of custom widgets. #498.