R1:d9d0daecd7fd

Scroll the chat column to the bottom when images load

Summary:
Fixes T7558. This might not be 100% perfect but should solve most of the issue.

I briefly looked at things like `MutationObserver` (some fancy next-gen browser junk) but couldn't immediately get it working.

Other methods for handling this kind of thing involve polling, complicated polyfills, etc. We could give `MutationObserver` a more serious effort if this is too leaky.

Test Plan:
- In a thread with some images, reloaded the page and…