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/redmine.json

@@ -1,24 +1,5 @@
 {
-  "releases": {
-    "6.0": {
-      "name": "6.0"
-    },
-    "5.1": {
-      "name": "5.1"
-    },
-    "5.0": {
-      "name": "5.0"
-    },
-    "4.2": {
-      "name": "4.2"
-    },
-    "4.1": {
-      "name": "4.1"
-    },
-    "4.0": {
-      "name": "4.0"
-    }
-  },
+  "releases": {},
   "versions": {
     "6.0.6": {
       "name": "6.0.6",