Instant Search: Add localization and formatting of all dates

[gibrown]

Pass the locale from the backend and use it to properly format the dates in the results and the date filtering.

@jsnmoon it looks like date filtering is not fully working (I think on master). Haven't figured out what broke it yet.

Fixes #13390

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.
• Add a Jetpack Search widget to the Search page sidebar and configure some date filters.
• Try searching and filtering.
• Try setting different locales. Can set it for the whole site or use a filter like so:

function jp_instant_search_options( $options ) {
	$options['locale'] = 'ko-KR';
	//$options['locale'] = 'en-GB';
	//$options['locale'] = 'ar-EG';
	return $options;
}
add_filter( 'jetpack_instant_search_options', 'jp_instant_search_options' );

[jsnmoon]

it looks like date filtering is not fully working (I think on master). Haven't figured out what broke it yet. (@gibrown)

I just pushed out a date filtering hotfix and rebased this branch on top of it.

[jsnmoon]

Looks good to go. I commented inline where we're beginning to accrue tech debt.