Use default JupyterLab CSS sanitizer options for Markdown

[krassowski]

Fixes jupyter-server/jupyter_server#577

6.4.1 removed all CSS styles from HTML inside of Markdown cells after moving to a maintained version of the sanitizer. This PR restores CSS styles in Markdown cells. For a test case of:

<span style="color: red; font-size: 30px">test</span>

Before:

After:

[krassowski]

The question is whether CSS styles should also be allowed in the output:

notebook/notebook/static/notebook/js/outputarea.js

Lines 692 to 695 in 2cfff07

	if (type===MIME_HTML || type==='text/svg') {
	var parsed = $(security.sanitize_html_and_parse(value));
	toinsert = append.apply(this, [parsed, md, element, handle_inserted]);
	} else {

I think that they should, just like they used to be and are in JupyterLab. Any thoughts?

[Zsailer]

I opened #6173 to follow-up on this question. In the meantime, I think we should merge this fix and cut a patch release to help unblock #6172.