Move append_output animation height lock release into timeout.

Firefox doesn't render images immediately as the data is available.  When animating the way that we animate, this causes the output area to collapse quickly before returning to its original size.  When the output area collapses, FireFox scrolls upwards in attempt to compensate for the lost vertical content (so it looks like you are on the same spot in the page, with respect to the contents below the image's prior location).  This is why @takluyver witnessed a page jump in #5128.  The solution is to wait a quarter of a second (seems reasonable in testing) after publishing the output to allow it to resize itself.

closes #5128