Re-instate HTML MIME type

[vidartf]

The HTML MIME type was removed in #2007. The agreed solution was that a shorter version should be re-instated after jupyter/notebook#2980 is fixed (ref #1777). The shorter version should probably:

• link to a more extensive explanation in the documentation
• or, include a collapsible section with the details (think <details><summary>Short version</summary><p>Long version here</p></details>)

[jasongrout]

To clarify, this is the HTML mime type in widget view display_data mimebundles.