API demo tweaks and demo server maintenance improvements

[EJMFarrow]

Appearance tweaks:

• A few formatting tweaks.
• Update for MathJax 3.
• Hide quiz files on API library page.
• Make drag and drop question fit slightly better on the page.
• Display the XML for non-STACK questions on XML edit page (stack.php). They were hidden before because we had quite a few multi-question files in the sample folder and we weeded out the irrelevant questions in those files. Now all our files are single question and HELM has quite a few description questions that shouldn't be ignored completely.

API demo server changes:

• I've had to create a separate Dockerfile and docker-compose for the demo server to maintain the SSL certificate on a restart. It also copies in the UoE footer. If the Docker container crashes it should restart with everything in place without intervention. On server failure we can relaunch from a snapshot. Documentation: https://www.wiki.ed.ac.uk/spaces/STACK/pages/758121847/STACK+API+Demo+server