Allow an instance to disable federation

[mouse-reeve]

Description

This allows an instance to turn off all contact with other ActivityPub services. The instance would still be able to connect to OpenLibrary and Inventaire. If there's existing federated data, there can be some odd behavior (like blocking an existing remote user will "work" but not propagate the block to the other server, and following that user would fail silently), but since I expect this to either be used from the outset or as a stopgap for some unforeseen issue, I didn't bother to clarify those situations in the UI (besides putting up a banner on the user page).

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What type of Pull Request is this?

• Bug Fix
• Enhancement
• Plumbing / Internals / Dependencies
• Refactor

Does this PR change settings or dependencies, or break something?

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[Flameborn]

Thank you for implementing this. If federation also includes book edits from remote instances, this might be a partial solution for #2841

[mouse-reeve]

It does prevent book data federation, but it’s not granular at all