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/apache-struts.json

@@ -1,33 +1,5 @@
 {
-  "releases": {
-    "7": {
-      "name": "7"
-    },
-    "6": {
-      "name": "6"
-    },
-    "2.5": {
-      "name": "2.5"
-    },
-    "2.3": {
-      "name": "2.3"
-    },
-    "2.2": {
-      "name": "2.2"
-    },
-    "2.1": {
-      "name": "2.1"
-    },
-    "2.0": {
-      "name": "2.0"
-    },
-    "1.3": {
-      "name": "1.3"
-    },
-    "1.2": {
-      "name": "1.2"
-    }
-  },
+  "releases": {},
   "versions": {
     "6.7.4": {
       "name": "6.7.4",