Future-proof the table ordering stuff

[davorg]

I've just fixed the mechanism that lets an author define a default order for the table on their page (see 14a4215). This had never worked (because it needed a column number and we were using names) and it had broken even more when I added another column without fixing the numbers in the JS config.

I'll be adding columns and maybe moving columns about in the future, so the current approach isn't scalable. We need to go back to allowing authors to specify their default sort column by name - and then have a hash to convert that to a number.

Maybe also think about storing the current sort order in a cookie.