Sanitize HTML to avoid 400 errors when posting disallowed attrs/tags

[parente]

e.g., <img> is not a supported tag. Adding an image in a markdown cell causes a 400. e.g. pandas added a stylesheet and style attributes between 0.19 and 0.21. Causes a 400 e.g. --- in markdown becomes <hr> but the storage format wants <hr />

Requires hooking into the md/html sanitization routines in nbconvert.

• Override markdown2html for jinja use and pass use_xtml to IPythonRenderer https://github.com/jupyter/nbconvert/blob/a96bfddbb9e4a9441ab37b5fd9d6236a5ee13a5b/nbconvert/exporters/html.py#L69 ()
• Add https://github.com/jupyter/nbconvert/blob/d9c66f54aff4380c3f8a3877421b7017139e7b64/nbconvert/preprocessors/sanitize.py as a preprocessor