System notification contains newline characters in message

## Expected Behavior

The `message` and `message_details` fields in the system event for system notification shouldn't contain leading or trailing newlines.

## Current Behavior

The `message` and `message_details` fields contain leading and trailing newlines.

```
$ http :9220/gl-system-events_0/_search | jq '.hits.hits[]._source | {message: .message, fields: .fields}'
{
"message": "NO_INPUT_RUNNING: There is a node without any running inputs\n\n",
"fields": {
"message_details": "\nThere is a node without any running inputs. This means that you are not receiving any messages from this\nnode at this point in time. This is most probably an indication of an error or misconfiguration.\n"
}
}
```

## Steps to Reproduce (for bugs)

1. See the command in "Current Behavior"

## Your Environment

* Graylog Version: 5.1.x