Better scrolling of long explain plans on explain.depesz.com

Thanks to some discussion on Slack, and work by Alexandre Felipe viewing of large explains will be now a bit easier.

You can see it, for example, in this explain.

First of all, when you scroll down, the column headers stay in place. Plus – you can always see the horizontal scrollbar to see the rest for really long node descriptions.

Thanks a lot, I appreciate the work.