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/weakforced.json

@@ -1,27 +1,5 @@
 {
-  "releases": {
-    "2.8": {
-      "name": "2.8"
-    },
-    "2.6": {
-      "name": "2.6"
-    },
-    "2.4": {
-      "name": "2.4"
-    },
-    "2.2": {
-      "name": "2.2"
-    },
-    "2.12": {
-      "name": "2.12"
-    },
-    "2.10": {
-      "name": "2.10"
-    },
-    "2.0": {
-      "name": "2.0"
-    }
-  },
+  "releases": {},
   "versions": {
     "2.12.1": {
       "name": "2.12.1",