Include sponsor page & rotating sponsor banners

[angelos]

We now only have a banner for our main sponsors. For our venue-sponsors, we will need a rotating set of banners on each page, and a sponsor page that shows all the current sponsors. I'm happy with not having this as a CMS feature, but having a set of static images plus some json for the links, that require a commit and redeploy when changing; the set of sponsors doesn't change all that often after all.