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 saw the scrollbar stay at the bottom.
• Tested with and without USB devices attached.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7558

Differential Revision: https://secure.phabricator.com/D12191