Feature: support static widgets in Reveal.js slides

[agoose77]

This restores the embedding of widget state in rendered notebooks using the SlidesExporter with the default reveal template. This has been implemented by adding a call to super() in the footer block, which calls the lab/base.html.j2 implementation.

I don't know whether this is the preferred style of nbconvert. Looking at base.html.j2 and index.html.j2, it seems like there is an implementation (base) vs client (index) separation. If this is the case, perhaps a more robust (although breaking) change would be to rename the footer block in base.html.j2 to something like embed_widget_state:

nbconvert/share/jupyter/nbconvert/templates/lab/base.html.j2

Lines 262 to 270 in e49b9a8

	{%- block footer %}
	{% set mimetype = 'application/vnd.jupyter.widget-state+json'%}
	{% if mimetype in nb.metadata.get("widgets",{})%}
	<script type="{{ mimetype }}">
	{{ nb.metadata.widgets[mimetype] | json_dumps }}
	</script>
	{% endif %}
	{{ super() }}
	{%- endblock footer-%}

[MSeal]

Thanks for the improvement, and sorry no one looked at this for a while