Permissions for author and contributor need a review and unit test coverage

[naz]

This issue is a follow up for #10214 (comment)

We need to review and cover with unit tests the differences between Author and Contributor roles permissions in authors models permissions: https://github.com/TryGhost/Ghost/blob/master/core/server/models/relations/authors.js#L321-L333. Specifically there should be a clarification on whether there's a difference between user being a co-author or a primary author.

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Additional tasks:

• update docs accordingly https://docs.ghost.org/faq/managing-your-team/#user-roles

[kirrg001]

FYI: We have many, many unit tests in the post_spec.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.