Convert the health checks to the Moodle Check API

[brendanheywood]

https://github.com/maths/moodle-qtype_stack/blob/master/adminui/healthcheck.php

this is a good candidate to convert over:

https://moodledev.io/docs/5.0/apis/subsystems/check

[sangwinc]

Brendan, thanks! I didn't know about this feature of Moodle. @timhunt are you familiar with this component of Moodle? It would be sensible to have at least one check if we can. Quite often colleagues don't install Maxima at all and then wonder why the plugin doesn't work, even though it has installed.

[timhunt]

Sorry, I don't know about that either.