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/sourcegraph.json

@@ -1,15 +1,5 @@
 {
-  "releases": {
-    "6": {
-      "name": "6"
-    },
-    "5": {
-      "name": "5"
-    },
-    "4": {
-      "name": "4"
-    }
-  },
+  "releases": {},
   "versions": {
     "6.5.2654": {
       "name": "6.5.2654",