Default scroll length closer to expected (i.e. browser default).

`article_scroll_down` and `article_scroll_up` scroll the page far more than the regular arrow buttons do on other pages. For me this is a surprising behavior and thus negative for the UI.

The "magic" value of 40 (pixels? Most likely) I put in corresponds to Chrome's and Opera's default scroll lengths for the arrow keys on my system at least. The earlier code was dependent on `ctr.offsetHeight/3`, which qualitatively must have resolved to a value of ~300 here. It made it difficult to maintain article context when scrolling.