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/jquery.json

@@ -1,15 +1,5 @@
 {
-  "releases": {
-    "3": {
-      "name": "3"
-    },
-    "2": {
-      "name": "2"
-    },
-    "1": {
-      "name": "1"
-    }
-  },
+  "releases": {},
   "versions": {
     "3.7.1": {
       "name": "3.7.1",