sources/community: Restore support for datasets/narratives stored with Git LFS

[tsibley]

This partially reverts «Remove an extra layer of CORS-unfriendly indirection from "raw" GitHub URLs» (d5ad654b) by switching this source's base URL back to github.com instead of raw.githubusercontent.com.

It's important to go thru github.com and let it redirect us to the final location because GitHub serves LFS blobs on a different final domain, media.githubusercontent.com.

Related-to: https://github.com/nextstrain/nextstrain.org/pull/658 Related-to: https://support.nextstrain.org/agent/nextstrain/nextstrain/tickets/details/668036000003625001

Testing

• [x] Broken community datasets now work
• [x] Checks pass

[tsibley]

Will merge as a hotfix after CI passes. (It works locally.)