load mathjax from the sever instead of cdn.jsdelivr.net

In frontend/public/index.html you load mathjax from cdn.jsdelivr.net. That has some disadvantages:

• you can't run the project in contexts without internet access eg. intranets, restrictive firewalls
• you depend on cdn.jsdelivr.net to be available
• it's not striclty GDPR conform because it transfers personal data (ip adress) to an outsider

Put it to all the other frontend dependencys.