Fix an issue that caused `ThreadList` to sometimes have an incorrect (too small)
internal `scrollContainerHeight` state after the app loaded, resulting in not
enough annotation cards being rendered for the height of the sidebar.

`ThreadList` measures the height of the scroll container after the
initial render in a `useLayoutEffect` callback. It then registers handlers for
window resize events in a `useEffect` callback order to re-measure the height
when it changes.

The sequence of events that lead to the incorrect state was:

 1. The sidebar iframe starts out hidden, by a `display: none` on a container of
    the iframe in the parent frame.
 2. When the `useLayoutEffect` callback runs, the iframe is hidden and so the
    scroll container height is measured as zero.
 3. The iframe is then shown and a `resize` event is fired at the window
 4. The ThreadList's `useEffect` callback runs and registers a handler for the
    window's `resize` event

Because the initial window `resize` event is fired before the listener is
registered, the scroll container height did not get re-measured.

The fix is to combine the `useLayoutEffect` and `useEffect` callbacks from steps
(2) and (4) so that the `resize` listener is registered immediately after the
initial size is measured. Combining the effects is also beneficial from a
readability perspective as it locates all the related state and logic together
in the component.

Fixes https://github.com/hypothesis/client/issues/3915