Instant Search: Add custom taxonomy filtering

[jsnmoon]

Changes proposed in this Pull Request:

This enables filtering by custom taxonomies using search widget checkboxes.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

Yes, this adds support for custom taxonomy filtering to Jetpack Instant Search.

Testing instructions:

• Add define( "JETPACK_SEARCH_PROTOTYPE", true ); to your wp-config.php.
• Ensure that your site has the Jetpack Pro plan and Jetpack Search enabled.
• Ensure that your site has configured a custom taxonomy. An example code snippet follows how one might register a custom taxonomy for your site.
• Add a Jetpack Search widget to the Search page sidebar.
• Add a custom taxonomy filter to the Jetpack Search widget.
• Enter a query into a search widget. Alternatively, navigate to a search page like /?s=privacy.
• Ensure that you can select a custom taxonomy filter checkbox.
• Ensure that other search filters, like dates and post types, still work as expected.

Configuring a custom taxonomy:

This following code snippet, when added to wp-content/mu-plugins/functions.php, will register a custom taxonomy named Speakers.

function register_speakers_taxonomy() {
    register_taxonomy( 'speakers', array( 'post' ), array(
        'label'    => __( 'Speakers' ),
        'template' => __( 'Speakers: %l.' ),
        'helps'    => __( 'Separate speakers with commas.' ),
        'sort'     => true,
        'args'     => array( 'orderby' => 'term_order' ),
        'rewrite'  => array( 'slug' => 'speakers' ),
    ) );
}
 
add_action( 'init', 'register_speakers_taxonomy' );

Proposed changelog entry for your changes:

None.

[gibrown]

There is some bugginess in the display here for non-English characters, but we have other places we need to fix up filtering display bugs also, so let's get this merged and then fix them all up.

https://cloudup.com/cdZFwcZw33R