Instant Search: Clean up some design bugs/issues

[gibrown]

Implements a number of design tweaks that were suggested from the demo:

• Gets rid of having multiple icons, but does potentially show a single one depending on the content or post type of the result. Hopefully this helps differentiate different types of results while not adding as much clutter.
• Fixes html for the filters so it is all divs
• adds left justify to fix issues with the zerif-lite theme
• a bit of spacing between filter checkbox and the label
• min-height for the results to try and prevent the page bouncing too much when you clear the search

Fixes some of the issues in #13391

Testing instructions:

• Add define( "JETPACK_SEARCH_PROTOTYPE", true ); to your wp-config.php.
• Ensure that your site has the Jetpack Pro plan and has Jetpack Search enabled.
• Add a Jetpack Search widget to the Search page sidebar. (though I guess it should now work without this added at all)
• Enter a query into a search widget. Alternatively, navigate to a search page like /?s=privacy.
• The following code lets you test against any other site by adding this filter and then adding &blog_id=20115252 to your search page url (that is the jetpack.com blog_id). Also can dynamically set the theme with &theme=twentyten

function jp_instant_search_options( $options ) {
	if ( $_GET['blog_id'] ) {
		$options['siteId'] = (int) $_GET['blog_id'];
	}
	return $options;
}
add_filter( 'jetpack_instant_search_options', 'jp_instant_search_options' );

function filter_theme( $theme ) {
	if ( $_GET['theme'] ) {
		$theme = sanitize_key( $_GET['theme'] );
	}
	return $theme;
}
add_filter( 'stylesheet', 'filter_theme', 1 );
add_filter( 'template', 'filter_theme', 1 );

[gibrown]

props @joanrho and @crunnells for a lot of the design feedback :)

[jsnmoon]

Changes look good 👍