Add django-health-check to web package

This allows for more details on whether the application is behaving well. One can test it by doing (formatting with jq is optional).

$ curl -sX GET localhost/ht/?format=json | jq
{
  "Cache backend: default": "working",
  "CeleryPingHealthCheck": "working",
  "DatabaseBackend": "working",
  "DefaultFileStorageHealthCheck": "working",
  "DiskUsage": "working",
  "MemoryUsage": "working"
}

django-health-check doesn't support newer than django 3.2 or python 3.8, but that is what we're using.