Implement the ability to switch between the completions and chat completions API

[artmatsak]

We've switched to the new chat completions API in 3822e35. Upon closer look, though, the bot performance now seems to be worse than with the originally used text-davinci-003 model.

I think we need to switch back to text-davinci-003 by default, but have a simple way to switch over to the chat completions API for experimentation.