Use External URLs

The original theme had iterated only over local resources and did not accept remote URLs. As of this writing, I saw that a couple of closed issues that had asked for this specific feature.

With this PR, we override and extend the original gallery so that a index.md can include external_images as a frontmatter. By making the R2 database public, I've removed the outdated step of uploading and downloading the photo assets per build. This approach may have to change in the future when I seek to protect my pictures and rights.

Another lesson learned: use wrangler locally to upload to R2 directly from the CLI. I didn't realize the dashboard had a batch limit.