fix(guest-author): enqueue the guest author admin script selectively

[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:

The guest author script (#3277) should only be enqueued where applicable.

How to test the changes in this Pull Request:

1. On trunk, observe the newspack-plugin/dist/other-scripts/co-authors-plus.js JS script is loaded for any admin request (e.g. editor)
2. Switch to this branch, observe it's not anymore
3. Add or edit a user - observe the script is included there

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?

[matticbot]

🎉 This PR is included in version 5.4.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 5.5.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀