Since #306 which set the height of the `<hypothesis-app>` container
element to 100%, the height of `document.body` is now equal to the
window's `innerHeight`. As a result, `document.body.clientHeight` now
returns the same value as `window.innerHeight` and `scrollOffset()`
returns 0.

Fix this by using `scrollHeight` instead of `clientHeight`, which takes
into account the height of all of the body's descendant elements,
including the `<thread-list>`.

This calculation would still return the wrong result if the
`<thread-list>` were ever placed in a separate scrolling container
within the sidebar app window, but that is an acceptible limitation for
now.