[web] Keep chat window scrolled to bottom on new messages
[web] Keep chat window scrolled to bottom on new messages

Summary:
@Adrian noticed a bug where he had to refresh the website to see some new messages. I think the new messages were actually there, but not visible because we don't currently adjust scroll position when a new message comes in.

I assigned @KatPo to review since she is currently working on `ChatMessageList` already, but @palys-swm may have more context on the `getSnapshotBeforeUpdate` [lifecycle method](https://reactjs.org/docs/react-component.html#getsnapshotbeforeupdate).

Reviewers: KatPo, palys-swm

Reviewed By: KatPo, palys-swm

Subscribers: KatPo, Adrian, zrebcu411, palys-swm

Differential Revision: https://phabricator.ashoat.com/D71

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