[feature] Distribute Accepts to followers; process Accepts of remote interactions

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

This pull request updates distribution of Accept activities for interactions on statuses, to send the Accept also to the accepter's followers, and CC it to public as well.

The PR also adds code to process incoming Accepts of interactions that target remote statuses.

The goal here is to ease interoperability with servers that don't yet know about interaction policies and therefore can't append the "approvedBy" field to statuses when sending them out. We can use the receipt of an Accept from an interaction-policy-aware instance to go and dereference the approved status from a non-interaction-policy-aware instance, and add the approved_by_uri to the db model ourselves. This should lead to fewer perceived gaps in followers-only conversations taking place between Mastodon (and other) servers and GtS servers, from the perspective of a GtS server.

Also updates the interaction policy documentation to mention this type of addressing.

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• 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.

[NyaaaWhatsUpDoc]

good work comrade 😎 🫡