Add support for WordPress Multisite

[rafaucau]

Resolves #100

• Update table prefix Reset Database instance after blog switch
• Ability to create models that have shared tables across the multisite network
• Add Multisite models - https://codex.wordpress.org/Database_Description#Multisite_Table_Details

[rafaucau]

When I use:

add_action('switch_blog', function () {
    $this->setTablePrefix($this->db->prefix);
});

This only works partially. It works correctly in the model's getTable() method, but then there's an error when executing the query:

Fatal error: Uncaught Exception: Table 'wordpress.wp_2_wp_2_posts' doesn't exist in wp-orm/src/Orm/Database.php:395

I can't find the place that could be causing generation of a double prefix after executing the setTablePrefix() method. It's not the table method or other similar ones in the Database class.

I'll reset the entire instance:

add_action('switch_blog', function () {
    self::$instance = null;
});

This works correctly.

[dimitriBouteille]

Hi @rafaucau

Thank you for your contribution!

I made 2 small feedbacks. Before validating the PR, I want to take the time to test and write some tests.

I hope to validate this RP in February for the 4.2.0.