List diamond and platinum sponsors on the sidebar in blog pages

[alexgmin]

Comment separated into a new issue:
#2302 (comment)

I don't know if this should be a separate issue or not, but I also need the Diamond and Platinum sponsors listed on the news page. I think it would be best to have them under the upcoming events but above the Archive.

This is a lower priority than adding the Diamond, Platinum, and Gold sponsors to the Community and About Pages.