HTMLExporter: Add a new output filter that excludes widgets if there is no state

[martinRenou]

This allows showing a static mimetype if the widget state is not available.

Example 1: Widget state not available, we render a static plot

Example 2: Widget state available, we render the widget (blank because the plot data is not part of the state (yet))

This has no implication on Voila, voila continues rendering the widgets properly (this is because it will simply not use this filter):

This is related to the work done in matplotlib/ipympl#343

[SylvainCorlay]

@jasongrout do you think we could consider this for 6.2?

We just released ipympl 0.8 which now includes a PNG representation of figure widgets. This fallback mechanism will allow nbconvert to render static Matplotlib figures when the widget state is missing, like GitHub does.

Note: this is solving a longstanding issue and request from the community, so I am very happy about it.

[maartenbreddels]

Very nice! I think this makes total sense since if the state is not there, nothing can be done with the mime bundle entry. A potential future issue could be when the widget state is stored outside of the notebook, not something to worry about now.

[jasongrout]

@jasongrout do you think we could consider this for 6.2?

I'm planning on releasing 6.2 tomorrow since there haven't been bug reports on the RC. I think this is a big enough change I'd rather do the release (rather than reset the RC timeline) and get it out the door, and immediately start the process for 6.3.

[SylvainCorlay]

Version 6.2 has been released a few days ago, and we haven't seen major breakage.
I think it is fine to start merging things in preparation for 6.3.