0.4.x: Fix markdown rendering logic

[martinRenou]

Using the following code:

from ipywidgets import Output

from IPython.display import display, Markdown

out = Output()

with out:
    display(Markdown("[google](https://google.com)"))

out

Results in not rendering the link properly, instead we see the raw code inlined "[google](https://google.com)"

This PR fixes that issue.

[github-actions]

👈 Launch a Binder on branch martinRenou/voila/fix_markdown_render

[martinRenou]

The PR correctly fixes the example above, but it seems to break mathematical expressions:

I also see this issue on Voila 0.5.x so this looks like a regression from updating to this new logic of handling mathjax.

[martinRenou]

This is working now 🚀

[jtpio]

Thanks!