This PR changes how the CF thumnail worker gets the config for a chart. It used to be the case that it would fetch the HTML file of the grapher page at the given slug and extract the config from that HTML. Now it looks up the grapher config json file in an R2 bucket.

This PR only changes this for published charts accessed by slug. A later PR will enable this also by UUID.

Cloudflare is a bit weird with the intersection of support of various features between CF workers/pages functions, R2 and local/remote dev support:

• Initially I wanted to use bindings in CF functions. For local development, wrangler dev does not support using a remote R2 bucket which is annoying
• I then thought I'd switch to fetching data from R2 buckets via HTTPS instead of bindings, similar to what we do with our variable data/metadata.json files. I realized then though that this only works if the bucket itself is accessible publicly via HTTPS. I'd like to keep the R2 bucket internal and not expose it directly.
• My next approach was to try to use the S3 API in the CF pages function. This didn't work because the default AWS library doesn't work in CF functions and the simpler replacement libraries that I could find were very barebones and made you deal with XML manually
• So finally I decided to go back to bindings and accepting the somewhat annoying dev story