Co-Authors Plus: Improve how post counts are managed for users

[danielbachhuber]

Currently, the activation hook for Co-Authors Plus doesn't do anything useful.

Ideally, when Co-Authors Plus is first used, it should create taxonomy terms for all of the users, properly set associations with existing posts, and update the post count for each author.

We'll need to do this in a way that works for sites with large number of posts and/or users.

[danielbachhuber]

There's a better approach for this: let's produce all of the data on the fly.

Basically, we need to do two things.

• Anytime we're getting the post count for an author, we need to fall back on WP default if the author doesn't exist as a term.
• When we're creating or updating a co-author term, we need to count both the number of term relationships they have as well as the number of post_author and then de-dupe.

[danielbachhuber]

In [479589]:

Co-Authors Plus: Only modify count_user_posts() when the author exists as a term object

See #1433

[danielbachhuber]

We may also need to unset the "posts" column in the Users WP List Table and do our own custom calculation...

[danielbachhuber]

In [479608]:

Co-Authors Plus: Provide our own custom post count as count_many_users_posts() in core Users WP_List_Table isn't filterable function anywhere but count_user_posts() is

See #1433

[danielbachhuber]

In [479620]:

Co-Authors Plus: Properly update the number of posts for each user by providing a callback for updating the post terms

Closes #1433