feat: automatically disable guest authors

[leogermani]

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:

After the introduction of the Guest Contributor role, we now want to be more proactive and disable CAP's guest authors if the site is not using the feature yet.

CAP's Guest Authors feature will be disabled by default if there are no Guest Authors in the site. If you want to force enabling it, add the NEWSPACK_ENABLE_CAP_GUEST_AUTHORS constant to your wp-config.php file.

How to test the changes in this Pull Request:

1. In a site that has some Guest Authors
2. Enable this branch and confirm the feature is still enabled and working
3. Check that the option was set: wp option get newspack_check_site_has_cap_guest_authors should return yes
4. Delete all your guest authors (or start with a new site)
5. Make sure to delete the option: wp option delete newspack_check_site_has_cap_guest_authors
6. Load any page of the site again
7. Confirm Guest Authors are disabled and you see our replacement page instead
8. Confirm the option now has the value no
9. Add define( 'NEWSPACK_ENABLE_CAP_GUEST_AUTHORS', true ); to wp-config
10. Confirm CAP's Guest Authors are re-enabled

Testing CRON

1. Repeat steps 1 to 4
2. Run wp cron event run newspack_re_check_guest_authors
3. Confirm Guest Authors are disabled and you see our replacement page instead
4. Confirm the option now has the value no

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?

[adekbadek]

Works as described, but I wonder if the migration won't be too involving.

[adekbadek]

typo

Suggested change

	// Make sure we check again if the site has guest authors evey hour.
	// Make sure we check again if the site has guest authors every hour.