Part of [ENG-3976](https://linear.app/comm/issue/ENG-3976)
This diff uses `react-window` package to render only visible threads. `react-virtualized-auto-sizer` is the recommended utility to automatically set height of the `VariableSizeList`.
There's one issue that I'm aware of: if you scroll down, the search bar will stop rendering and lose focus (but it will keep it's contents). On 1-1 with @ashoat we decided that's good enough for now. I will create an issue explaining a possible solution with docking the search bar before landing this diff.
Adding ashoat as reviewer because of new dependencies.