Get notification permission on install

Notifications are still configured off by default, but now we don't need
to trigger the pop-down permission dialogue if the user enables them.

manifest.json

@@ -7,7 +7,8 @@
     "offline_enabled": false,
 
     "permissions": [
-      "unlimitedStorage"
+      "unlimitedStorage",
+      "notifications"
     ],
 
     "icons": {