index.html: Add missing class for builders table header row
Add individual build times to web interface builder table

The individual build times are now passed to the web interface.
Additionally, the 2-second updates are way too frequent in my opinion.
The .data.json files can be 400k compressed, and 1.5M uncompressed.  That's
too big for such frequent pulls.  For now, I've changed this to a period of
5 seconds.