Markdown output not shown for Untrusted Notebooks

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I'm using `IPython.core.display.Markdown` to dynamically generate markdown content in a notebook. When I use a tool to update the notebook, such as `jupyter nbconvert`, the Notebook may become [untrusted](https://jupyter-notebook.readthedocs.io/en/stable/notebook.html#trusting-notebooks) for the server. In that case it only shows the `text/plain` part of the output.

In the console I am seeing `Ignoring untrusted text/markdown output.` The [code](https://github.com/jupyter/notebook/blob/b8b66332e2023e83d2ee04f83d8814f567e01a4e/notebook/static/notebook/js/outputarea.js#L692) shows that it would sanitize HTML but not Markdown. I understand that Markdown can be converted to HTML and that it itself can contain HTML. Here I would expect it to be able to sanitize Markdown as well (e.g. by conversion to HTML and displaying the sanitized HTML).

The cells with a Markdown type seem to display fine. Not sure whether they get sanitized.

Start in notebook/static/notebook/js/outputarea.js around the linked untrusted-output handling, and reproduce the case with IPython.core.display.Markdown in a notebook made untrusted by nbconvert. Compare it with how notebook Markdown cells are displayed. Done means untrusted Markdown output is safely sanitized and shown instead of only its text/plain part.

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