[bugfix/chore] Announce reliability updates

[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.

• Use per-URI deref locks for Announce dereferences to avoid races.
• Use new BoostOfURI field on statuses for doing Announce derefs (new field not stored in the db).
• Ensure faves and boosts are redirected properly to top-level status when the fave or boost was created targeting the ID of a boost wrapper.
• Don't serve boost wrappers via the web UI or the fedi API.
• Use Unretrievable to do noop nicely when a boost target is on a blocked domain.
• Fail to serialize boost via the client API if BoostOfID is set but BoostOf is not (for whatever reason).

This is rather a hodge-podge of fixes and updates. It MAY resolve #2394 but we'll have to run it for a while to see, since I couldn't actually definitely identify the root cause of that bug.

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 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.