Add topics to custom set. Fixes #937 (#948)

* Add topics to custom set. Fixes #937

* Add topics to custom set. Fixes #937
* Add generics to list of topics for custom set

config.json

       "core": false,
       "difficulty": 10,
       "slug": "custom-set",
-      "topics": null,
+      "topics": [
+        "sets",
+        "equality",
+        "generics"
+      ],
       "unlocked_by": "linked-list",
       "uuid": "377fe38b-08ad-4f3a-8118-a43c10f7b9b2"
     },