Rewrite [img] tags to use our img proxy

[danneu]

Working on a proxy here: https://github.com/danneu/guild/tree/master/img-proxy

We'll be able to rewrite image tags to use our proxy:

GET https://proxy.roleplayerguild.com/?url=https%3A%2F%2Fexample.com%2Fflower.png

It'll save the image in Cloudflare R2. This way the images in old posts don't break over time.

Once we've tested it out in production on new posts, we can rewrite old [img] tags to use the proxy to keep as many old images alive as possible.