Allow responding to chatgpt answers

[Nikurasuu]

In order to have full conversations with the chatgpt command, the user had to copy and paste the old messages to keep the information needed for the full conversation.

The ChatGPT API allows to keep track of conversations by adding the parent message id to the prompt. But this parent message id needs to be stored somewhere together with the discord message id, in order to keep track of conversations.

Another more simple way would be to just copy the message the user is responding to and just add it before to the prompt, but this would probably use more tokens as the prompt gets bigger.