Add a public API for Colossal Fund ads

[dfabulich]

The IFDB home page https://ifdb.org/ now has a donation button for IFTF, but it's not very good.

Instead, I wish we could run an ad for the Colossal Fund, including the goal bar + links.

I think it would make sense to put this ad not just on IFDB's home page, but also on https://ifarchive.org/, on https://iftechfoundation.org/, etc.

So, I think I want to request a public API for Colossal Fund ads. I'm imagining it'd be an URL that returns an HTML fragment, suitable for inclusion on other sites. IFDB would request that URL on our backend, caching it for N minutes (how long?), and we'd present the ad on our home page.