[bugfix] Add proper constraints on status faves, dedupe

[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:

• Introduces proper constraints on status faves -- now, an account can only like a status once.
• Includes a migration to delete duplicate status faves, before adding constraints to status fave table.
• Implements federating undo logic for status likes -- we didn't have this before for some reason???
• Does some v. small tidying up in the bits of the codebase that had to be touched ✋

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. -- tested the migration on both sqlite and postgres, seems OK
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.