Improve scrolling

[neilrqm]

Currently the whole page scrolls, which a) hides the header row controls (activity selection, copy link button, etc.) and b) scrolls the chart offscreen. All we really want to scroll is the area selection div. The header should stay at the top of the screen, and the chart should not scroll. This might be obviated by #7 since the length of the area selection list will shrink. However it is probably worth implementing regardless, as the area selection div is quite short on mobile in horizontal mode.