Disable link outs for private datasets

[tsibley]

arose from conversation with CDC

Datasets in private groups are not available for viewing on other platforms: they require CORS requests made with credentials (in this case, HTTP cookies), and 1) we deny credentialed CORS requests and 2) most external services are making uncredentialed requests (because they're more likely to succeed).

We should disable the link outs with an appropriate explanation. Possibly similar to how the Nextclade link out is disabled when the dataset is not compatible.

This will require coordination with Auspice, or potentially a larger redesign of how link outs work such that we move them completely out of Auspice itself and into a nextstrain.org customization of it.

Separately, we could consider options for opening up private groups to link outs/integrations, but that's a larger conversation with more implications to consider.