[web] Fix small thread list scrolling issue

Summary:
Just noticed that if you put your mouse cursor on the thread list in a place where the scrollbar normally displays you won't be able to scroll because there will another top-level scrollbar (for some reason on production I can only reproduce it in cases where the threads don't fill the whole list).

This was introduced when I landed ENG-3976 because the thread list container now contains the react-window list and the scrollbars overlap each other. This is a quick fix that sets overflow: hidden on the container (we don't want to scroll it, the scrolling is handled by the inner list).

Test Plan: Try the reproduction mentioned in the summary and check if the scrolling works and the second scrollbar doesn't appear

Reviewers: inka, patryk, rohan, ginsu

Reviewed By: patryk

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8727