Edit rather than Append

[zalo]

@rossimo A few ideas that might help this to feel really polished :-)

1. Ideally there's an option for the bot to edit the original message rather than append a new one.
   -- This is similar to how Midjourney's bot edits messages as their Image Diffusion AI converges on the final output.

2. Ideally there could be an option for the images being self-served
   -- Midjourney swaps out the image from the underlying AWS Bucket URL, which is sort of rate-limited, but since we're self-hosting, we can update .gifs a lot more frequently.

3. Use the edits to "stream" the .gif updates in, once per duration of .gif.
   -- This should resolve the ambiguity of how long to keep each .gif's duration. If they're always continually swapped out every 1-2 seconds, then the delay will be the only issue.

4. Use .webp to save bandwidth and improve framerate?
   -- This one is dubious, since iPhones still don't have the ability to play .webp's yet, but it might help in some universe (and it's typically dead-simple to encode them from Google/Chromium based runtimes).