Redo with promises

Should I redo the earlier bot examples with promises? It's tricky b/c I'd like that to be available, but I used callbacks in the videos so would make it hard to follow along. Maybe I should make both available!

Review the earlier bot examples and the related videos first, focusing on where callbacks are used and how a promise-based version would affect the walkthrough. Clarify whether both callback and promise examples are needed; done when the agreed examples are available without making the existing video follow-along confusing.

索引モデルが issue の本文から書いたものです。