Adding topics to problem 'Prime Factors' (#885)

* Adding topics to prime factors

* Adding math as problem area

* Removing recursion as a topic

config.json

       "core": false,
       "difficulty": 5,
       "slug": "prime-factors",
-      "topics": null,
+      "topics": [
+        "arrays",
+        "conditionals",
+        "integers",
+        "lists",
+        "loops",
+        "mathematics"
+      ],
       "unlocked_by": "triangle",
       "uuid": "599c08ec-7338-46ed-99f8-a76f78f724e6"
     },