Log screen should pin itself to the bottom of the scroll area

[liamhuber]

The log screen introduced in #123 can get quite long, and it would be great to have it automatically start at the bottom of the scrollable area (and pin itself there when it's at the bottom and more content gets added). Unfortunately this behaviour is not currently supported by ipywidgets. Since this is a relatively minor QoL thing, I'll probably just wait and hope that it gets supported upstream so that the fix is just one or two extra layout lines or whatever.