Remove invalid releases automatically (#466)

Automatically remove empty releases or releases which are released in the future, regardless of the product.

Also refactored a bit releasedata.py to use better names and clarify types.

releases/nvm.json

@@ -1,33 +1,5 @@
 {
-  "releases": {
-    "0.40": {
-      "name": "0.40"
-    },
-    "0.39": {
-      "name": "0.39"
-    },
-    "0.38": {
-      "name": "0.38"
-    },
-    "0.37": {
-      "name": "0.37"
-    },
-    "0.36": {
-      "name": "0.36"
-    },
-    "0.35": {
-      "name": "0.35"
-    },
-    "0.34": {
-      "name": "0.34"
-    },
-    "0.33": {
-      "name": "0.33"
-    },
-    "0.32": {
-      "name": "0.32"
-    }
-  },
+  "releases": {},
   "versions": {
     "0.40.3": {
       "name": "0.40.3",