Allow sort by "updated_at"

[kyungmin]

There has been a dashboard request on being able to sort transactions by most recent change: balanced/balanced-dashboard#1578

However, sorting by updated_at is currently not supported by the API:

  "errors": [
    {
      "status": "Bad Request",
      "status_code": 400,
      "description": "Unable to sort on unknown field \"updated_at\" Your request id is OHM66e21f9487ae11e49c920230f00c9248.",
      "request_id": "OHM66e21f9487ae11e49c920230f00c9248"
    }

[mjallday]

search index does not include updated_at as a field so not trivial to sort this in ES. we'd need to add updated_at as a field that we index.