Backport wp_collaboration table

[chriszarate]

What?

Add wp_collaboration table and WP_Collaboration_Table_Storage implementation.

Why?

Backport of WordPress/wordpress-develop#11256

Note that this will likely be out of date as additional changes are made to this pull request.

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Co-authored-by: chriszarate <czarate@git.wordpress.org>
Co-authored-by: josephfusco <joefusco@git.wordpress.org>

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[josephfusco]

Hey @chriszarate, 3 changes from the last review on WordPress/wordpress-develop#11256 to pick up here:

1. set_awareness_state - SELECT now fetches id, date_gmt. Returns early if timestamp unchanged. WHERE uses $exists->id instead of $exists.
2. add_update - Adds 'user_id' => get_current_user_id() with '%d' format.
3. Cleanup when disabled - Just unschedule cron and return. Skip the DELETE queries.