Add "Show rules" checkbox and attendent css and js

Add "Show rules" checkbox and attendent css and js

The only problem here is that as the column headings stick to the
page their background covers only the columns, not the entire
width of the screen/enclosing div.  Which means that when background
rules are displayed they are uncovered in the horizontal area
where the table headings appear.  The backround_rules.css
_could_ apply the rules to the table instead of the div, but then
the rules don't go all the way across the page.

There is surely a way to fiddle with this but it's not worth bothering
with now.