Check if audits is loaded before executing database queries

[wkirby]

This should be a no-op change that allows for performance improvements by preloading the audits association. By checking if the audits association is already loaded, we can avoid N+1 queries by operating on the already loaded audits in ruby instead of going to the database again to check if a version exists, etc.

This code should be functionally identical to the existing code, but allows for something like:

@posts = Post.all.include(:audits)

@posts_at_v1 = @posts.map do |post|
  post.revision(1)
end

To avoid repeated database access.

Fixes #719

[danielmorrison]

Tests are failing, but I like the idea.

[wkirby]

@danielmorrison thanks, will fix test failures

[wkirby]

@danielmorrison specs are now passing except for code coverage (see the test run on our fork here: https://github.com/apsislabs/audited/actions/runs/10538941999?pr=1). I believe this is because of the addition of the new loaded? branches being untested.

I'm happy to either:

• Update coverage exceptions so the tests pass
• Attempt to add some specs to cover these branches