Fix tabindex's in /2008/site/ design

[tripu]

Everywhere the 2008 design is used (eg, on the home page) there are 3 instances of tabindex imposing some tab order (1, 2, 3) that doesn't necessarily match the “visual” order of elements on the page. (Also, I wonder if tabindexes on specific pages would sometimes conflict with those three fixed ones in the chrome of the page.)

Detected by @LJWatson on w3c/wbs-design#49 (thanks!)

[LJWatson]

If someone uses tabindex with positive integer values in the content, it will very likely conflict with those in the template. Another good reason for not using tabindex in this way.