Create a new slash command to view the buns leaderboard.

* Recognize a slash command for `/buns`.
* Read all items in the buns table and sort them by `buns` value.
* Return the list in a response Slack message limited to top 5 results.
* Make sure to use canonical names for the leaderboard (not `enterprise_id`) by calling the utility method (#30) five times (one for each ID).