Support message editing/retries

[jmpaz]

See m1guelpf/chatgpt-discord#1 – basically:

• an LLM-powered chatbot accessed via a simple interface might be fun to play with, but without this its potential is severely limited
• OpenAI was well aware of this when they launched ChatGPT
  • while Poe is currently great in many respects, this functionality's absence is very noticeable
    • the same applies to this repo, and its frontend (and even Bing 😊)
    • no matter how good the prompts may be, implementing this is necessary for them to be maximally valuable