Feedback from Privacy review

Please see https://github.com/w3c/mathml/issues/576.

They have three main concerns although there are some other minor issues
such as with mglyph and content MathML's definitionURL (etc):
1. Privacy and Security appendices should not defer to core because MathML
4 has other features.
2. href in MathML 4
3. Intent issues dealing with privacy and a little bit about security. For
example, because "intent" is not visible, someone can hide things in the
intent such as harmful speech that only AT users deal with. My reading is
that they don't object to intent, but that the spec needs to say more about
them.

They mainly want appendix D4 and D5 updated/improved.

We will discuss this at an upcoming meeting.

    Neil