[bugfix] Don't copy ptr fields in caches

[tsmethurst]

Description

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

This PR fixes the following scenario:

• Some of my statuses get cached after database calls.
• My account model is attached to those statuses.
• I update my profile picture.
• The statuses still show the old profile picture, on both the web view and via the API.

Essentially, what was happening in the above scenario is that the old account model was being kept on the cached statuses, and those statuses weren't being invalidated when the account model was updated.

There are two possible approaches to fixing this:

1. Invalidate all cache entries that contain a pointer to a given account (or whatever) when that account (or whatever) updates; OR --
2. Don't return cached struct pointers when copying a cached model and giving it to the caller.

This PR takes the second approach, on the basis that it will lead to fewer database calls than the first approach, and preclude super complicated cache invalidation logic.

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 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]

i think the issue you're likely seeing is probably in the timeline cache rather than the database cache, given that the timeline pre-prepares a frontend model with all the extra parts fully populated.