[feature] Use local_only field, deprecate federated field

[tsmethurst]

Description

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This pull request updates the status create API and status API model to use the local_only field to indicate non-federated statuses. The local_only field is already in use by implementations such as GlitchSoc, so this should make things a little easier for client app devs. To avoid breakage, the federated field has been deprecated. It still works, but only if local_only is not set, and local_only takes priority.

Along the way, I also added checks for LocalOnly statuses to the visibility filter. We were already not serving unfederated statuses via the web frontend, but references to local-only posts were still being included in outboxes (not the posts themselves), and I think Public local-only posts were included in API responses if instance-expose-public-timeline was set to true. Not sure since I don't think anyone was actually using this combination of features.

Closes #1035

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.