add db health endpoint

jsc_custom/apihandler/__init__.py

+from . import db_health
 from . import incidents
 from . import misc
 from . import self_api_handler_oauth

jsc_custom/apihandler/db_health.py

+from jupyterhub.apihandlers import APIHandler
+from jupyterhub.orm import Role
+
+
+class DBHealthAPIHandler(APIHandler):
+    async def get(self):
+        Role.find(self.db, "any")
+        self.set_status(200)
+
+
+default_handlers.append((r"/api/health_db", DBHealthAPIHandler))