Messages should be in date (DESC?) order by default

[davewhiteland]

This is a wee bit horrible, because Cake lets the order be specified in the model, but there's a nasty gotcha in the way it then puts that in postgres (worked fine on the mySQL implementation).

This shortcoming is really starting to show now we've got more than a handful of messages in the system.

The workaround is to click on the (paginated) columns of course, but it's not really acceptable for there to be no defined order before that.