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/linuxmint.json

@@ -1,14 +1,5 @@
 {
   "releases": {
-    "lmde6": {
-      "name": "lmde6"
-    },
-    "lmde5": {
-      "name": "lmde5"
-    },
-    "lmde4": {
-      "name": "lmde4"
-    },
     "22.1": {
       "name": "22.1",
       "eol": "2029-04-30"
@@ -32,36 +23,6 @@
     "21": {
       "name": "21",
       "eol": "2027-04-30"
-    },
-    "20.3": {
-      "name": "20.3"
-    },
-    "20.2": {
-      "name": "20.2"
-    },
-    "20.1": {
-      "name": "20.1"
-    },
-    "20": {
-      "name": "20"
-    },
-    "19.3": {
-      "name": "19.3"
-    },
-    "19.2": {
-      "name": "19.2"
-    },
-    "19.1": {
-      "name": "19.1"
-    },
-    "19": {
-      "name": "19"
-    },
-    "18.3": {
-      "name": "18.3"
-    },
-    "18.1": {
-      "name": "18.1"
     }
   },
   "versions": {}