Override customizer and user edit screen instead of requiring patch outright

[sc0ttkclark]

Add hooks to auto-override customizer classes and user-edit.php for testing purposes.

Customizer manager class override:

remove_action( 'plugins_loaded', '_wp_customize_include' );
add_action( 'plugins_loaded', '_wp_fields_api_customize_include' );

User profile edit override:

add_action( 'load-user-edit.php', '_wp_fields_api_load_user_edit' );

[sc0ttkclark]

is_customize_preview() in wp-includes/theme.php will always return false because the class is no longer an instanceof WP_Customize_Manager:

/**
 * Whether the site is being previewed in the Customizer.
 *
 * @since 4.0.0
 *
 * @global WP_Customize_Manager $wp_customize Customizer instance.
 *
 * @return bool True if the site is being previewed in the Customizer, false otherwise.
 */
function is_customize_preview() {
    global $wp_customize;

    return ( $wp_customize instanceof WP_Customize_Manager ) && $wp_customize->is_preview();
}

Core will need a way for us to override this.

[sc0ttkclark]

Actually the above should be fine, our implementation uses WP_Customize_Manager and assumes the core class never gets loaded.

[sc0ttkclark]

Implemented in latest commits