WIP Fix error: Namespace prefix m on math is not defined

Note: Please do not merge.

I've created a string manipulation function `fixup_cnxml` to try to remove the error before we use lxml.etree.  Is this the right place to fix this?

I have thought of two ways to fix "Namespace prefix m on math is not defined":
 - as below, remove the prefix
 - add "xmlns:m="http://www.w3.org/1998/Math/MathML" to <content>.

I don't really know what's right.  They give different html outputs.