New spam protecting captcha filter installed

Peter

As you might have noticed we have been getting a lot of spam vandalism on CFD-Wiki in the last few days. I have just installed a captcha extension that will ask everyone who registers to compute a simple sum and enter the answer. This is to ensure that everyone who registers is a human being. An automated spam program will hopefully not compute the sum correctly and hence will not be able to automatically register and post spam vandalism. Jonas had this extension installed a year ago but after we upgraded to the latest software we didn't re-install it. But now that we see spam problems reocurring we have installed it again.