Proxy CORGIS datasets through server

[acbart]

There might need to be a more general interface for this down the road, but for now I'm pretty sure we want to start proxying the CORGIS datasets access through our own server. Not only does this make us nicer to GitHub, but it'll probably avoid some weird security errors involving cross-origin browser requests.

Shouldn't be too bad, just add another endpoint and some proxying/caching code to avoid repeated trips to the github site. I wonder what the best solution is, requests-cache? Or perhaps we can even just do all this on the Nginx side by proxying.