Utilize model class & add ability to create conversation with specific LLM

[Whitelisted1]

• Added the ability to create a conversation with specific llm instead of always using the self.active_model llm
• Instead of representing models as strings, they are represented in the model class. This allows us the get information about the model, such as the description

Example usage of the code changes:

chatbot.new_conversation(0) # create a conversation with the model at index 0

print(chatbot.active_model.id, chatbot.active_model.description) # print active model's id and description

[Soulter]

Good!