[bugfix] Don't return Account or Status if new and dereferencing failed, other small fixes

[tsmethurst]

Description

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This pull request fixes up some of our account and status dereferencing logic in the following ways:

• Don't return a near-zero Account or Status when it's new and a call to the remote instance returned a 4xx status code. Log + return an error instead. Previously, returning a partial account or status which hadn't yet been inserted properly in the database was causing all sorts of problems, since it would be put in the cache with zero values all over it. See [bug] [federation] honk (tedu) #2527 and [bug] Panic in AccountToAPIAccountPublic when looking up account from Catodon #2514.
• Derive published time for accounts using the published property. This should allow the actual remote creation time of the account to be stored, instead of just the time that the GtS instance first saw the account, which was misleading in that it was making some accounts look way newer than they actually are.
• Refactor some of the webfinger response parsing logic a bit to reuse existing regular expressions, add a new regular expression for the user web path (/@username).

Checklist

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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.