Add translation for checking shared secret

aleksis/apps/maka/frontend/messages/en.json

@@ -33,7 +33,8 @@
     "shared_secret": {
       "title": "Shared secret",
       "enter": "Shared secret needed for accessing page",
-      "error": "Wrong shared secret entered. Please try again."
+      "error": "Wrong shared secret entered. Please try again.",
+      "checking": "Checking for correctly entered shared secret…"
     }
   },
   "group": {