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/kubernetes-node-feature-discovery.json

@@ -1,24 +1,5 @@
 {
-  "releases": {
-    "0.17": {
-      "name": "0.17"
-    },
-    "0.16": {
-      "name": "0.16"
-    },
-    "0.15": {
-      "name": "0.15"
-    },
-    "0.14": {
-      "name": "0.14"
-    },
-    "0.13": {
-      "name": "0.13"
-    },
-    "0.12": {
-      "name": "0.12"
-    }
-  },
+  "releases": {},
   "versions": {
     "0.17.3": {
       "name": "0.17.3",