Strip out any remaining HTML as Kolibri will only render pure markdown.

[rtibbles]

Summary

Description of the change(s) you made

• Strips out any remaining HTML tags after we have done any other relevant conversion.

Manual verification steps performed

The issue was a little hard to reproduce without Microsoft Word and Windows on hand, but the simplest thing to do was to inspect the HTML of the editor area, edit it as HTML and then paste this into the HTML:

Karol <span style="letter-spacing: 0.03em;" data-tomark-pass="">__________________ talks in class</span>

then confirm that this gets saved without the HTML embedded.

Fixes #3345

[MisRob]

Thank you, @rtibbles, it's looking good to me.