Add sponsor logos to 'sponsors' and 'index' pages.

I don't know what the ideal location is for sponsor logos on the index page.  Right now they're in the flow of the rest of the page, shoehorned above the "Workshop Recaps" section.  I don't think that is a long-term placement, but for the current 4 sponsors it looks okay.

Feel free to merge (and deploy) or massage as appropriate.