implement conversation history for ChatGPT

[Kevin-Kwan]

• conversation history/memory should be stored in a database (tested with MongoDB at work, works fine) for each user instead of sharing a history across all servers and users
• probably looking into langchain for implementation since we can add more tools to make the bot more fun like access to the internet

[Kevin-Kwan]

reminder that we might need to implement a duplication check (old implementation used Levenstein algo) in case the bot thinks it can just fallback on memory if a question is similar to a past question