Use config schema published on dodona.be

The schema here actually contains errors (no intermediate names property under description. Since we will be hosting a schema on dodona.be anyway, just refer to that.

Author: chvp

package.json

@@ -20,7 +20,7 @@
           "config.json",
           "dirconfig.json"
         ],
-        "url": "./schemas/config.schema.json"
+        "url": "https://dodona.be/schemas/config.json"
       }
     ],
     "yamlValidation": [