Use same manifest for core + staging

[jameshadfield]

As discussed in slack [1]:

Previously, a big part of the reason we had a manifest_guest.json for staging, was to be able to test updates to manifest without worrying about breaking the server too badly.

This has some negatives if there are datasets missing on the staging bucket, but the positives greatly outweigh these (and staging is for internal use only, really).

[1] https://bedfordlab.slack.com/archives/C01LCTT7JNN/p1681784685980769

Testing

• [x] test review app for staging + core URLs

[trvrb]

Thanks for attending to this @jameshadfield. Everything looks good from my perspective.

This does surface issue with a few datasets present in manifest and absent from staging.nextstrain.org, but really don't think this is any worse than the current inverse situation with many datasets present at staging.nextstrain.org, but absent from the manifest.