add support for "poll ended" notfication type

example:

{
  "account": {...},
  "created_at": "2021-08-18T14:03:54.000Z",
  "id": "703382",
  "pleroma": {
    "is_muted": false,
    "is_seen": false
  },
  "status": {...},
  "type": "poll"
}