Users Menu: add new column listing the linked guest author.

[jeherve]

Fixes #494

The post count in the Users Menu changes when you've linked an account to a guest author. That is not always clear to everyone, as explained in #494.

If we add a new column to the menu that shows that guest author, right before the post count, I think it makes things a bit clearer.

Before

After

What do you think?

Testing

To test, go to Users > Guest Authors, create a few guest authors and link them to existing accounts on your site like so:

Then visit the Users menu and make sure the information and links displayed there are correct.

[rebeccahum]

@jeherve That's not a bad idea because it would mirror the Guest Author user panel and would be more consistent:

[GaryJones]

Only the last commit is relevant.

[jeherve]

I do not really contributo to Co-Authors Plus anymore, but I rebased that branch if that can make your job easier. The PHPCS error doesn't appear to be related to this PR though:

FILE: .../runner/work/Co-Authors-Plus/Co-Authors-Plus/co-authors-plus.php
----------------------------------------------------------------------
FOUND 1 ERROR AFFECTING 1 LINE
----------------------------------------------------------------------
 609 | ERROR | count_user_posts() is highly discouraged due to not
     |       | being cached; please use wpcom_vip_count_user_posts()
     |       | instead.
     |       | (WordPressVIPMinimum.Functions.RestrictedFunctions.count_user_posts_count_user_posts)
----------------------------------------------------------------------

-- https://github.com/Automattic/Co-Authors-Plus/actions/runs/5712043848/job/15474752693?pr=502