Fix images not showing in Jupyter Markdown tables
Previously cell attachments were ignored by the iPython notebook parser.
As described in
https://nbformat.readthedocs.io/en/latest/format_description.html, we
need to support the attachments
dictionary and use that to render
images.
Closes #18540 (closed)
Testing
Test iPython notebook: https://gitlab.com/gitlab-org/gitlab-test/-/blob/add-ipython-files%20/files/ipython/markdown-table.ipynb