Option to disable federation of nsfw instances

[dullbananas]

Requirements

• [X] Is this a feature request? For questions or discussions use https://lemmy.ml/c/lemmy_support
• [X] Did you check to see if this issue already exists?
• [X] Is this only a feature request? Do not put multiple feature requests in one issue.
• [X] Is this a backend issue? Use the lemmy-ui repo for UI / frontend issues.
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Is your proposal related to a problem?

Instance owners might not want to cooperate in providing access to porn or waste server resources for it.

Describe the solution you'd like.

Unless an option is manually enabled, don't federate with instances that have content_warning set.

Describe alternatives you've considered.

None

Additional context

No response

[Nutomic]

It should be enough to throw an error here if the site has a content warning. Then it wont be possible to fetch any communities from nsfw instances.

[dessalines]

I'm guessing we can't take advantage of the sensitive field for communities, since NSFW instances don't have all their communities marked that way.