Hotfix/migrate guest authors cli

[leogermani]

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Changes proposed in this Pull Request:

This PR changes the logic of the guest author migration script.

Basically, the script was checking if a guest user already existed as a regular WP user based on the user_login. I'm changing that to check for the user_nicename.

In a regular situation, the nicename will usually be the same thing as the login, but there are cases where they can be different. One of them is when you migrated users from another system and treated these fields in a different way in the database.

CAP uses the $user->user_nicename to create and match the author term, and that's why this is the field we should look at.

How to test the changes in this Pull Request:

1. Create a few guest authors and assign posts to them.
2. Run the script and make sure the guest contributors users are created and the posts are correctly assigned.
3. Create a guest author and make sure there is a wp user that has its nicename with the same value of the GA's login
4. Run the script and make sure no new WP user is created, and the posts that were assigned to that GA is assigned to this WP user

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?

[eddiesshop]

This seems to work as advertised from my testing. Just want to note though that after the first time I ran the script and the Guest Authors were converted to Users, I was unable to create another Guest Author with the same details because the system prevents the nicenames from being duplicated. So what I did instead was I added an extra character to the display name (which governs what nicename gets created), then manually went into the DB and updated the cap-user_login field for the GA to match the nicename of the previously converted user. The script worked just fine still, but if this level of intervention (manually updating values in the DB) isn't within the scope of testing please let me know.