Handle Slack rate limiting

The 'Post a message to a chat service' listener uses Slack's conversations.list endpoint to get every place that a message could be posted to. We use cursoring as recommended to get 100 conversations per request, but the endpoint is rate-limited to around 20 requests per minute. Thus Slack instances with more than about 2000 conversations will result in us receiving a 429 Too Many Requests error: we should handle these by backing off in accordance with the Retry-After parameter of the response.