feat(co-authors-plus): option to block the Guest Authors menu item

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Newspack discourages the use of Co-Authors Plus' Guest Authors feature and offers a way to migrate them to WP Users (#3068). This PR adds an optional (behind NEWSPACK_DISABLE_GUEST_AUTHORS feature flag) blocking of the "Guest Authors" menu item.

How to test the changes in this Pull Request:

1. Install Co-Authors Plus and visit the Users -> Guest Authors link, observe that Guest Authors can be added
2. Set NEWSPACK_DISABLE_GUEST_AUTHORS environment variable to true
3. Refresh the page, observe that the Users -> Guest Authors link leads to this message and Guest Authors can't be added here:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[leogermani]

You can disable guest authors by simply doing add_filter( 'coauthors_guest_authors_enabled', '__return_false' )

[adekbadek]

I know! The intention here is to say "use this instead of the thing you were used to", not just hide a feature and leave the user wondering why it disappeared.

[adekbadek]

Note: this PR will possibly be replaced by #3238

[adekbadek]

Superseded by #3277