Move is_robot function to javascript

The Liveblog is currently reyling on an existence of `vary_cache_on_function` function when modifying the behaviour for bots.

As not all environments taking advantage of page level caching have the posibility of varying page cache on user agent, the check should happen in browser.

This commit it moving the logic to inside the js/liveblog-lazyloader.js taking advantage of the same regex and disabling the lazyloading initialisation for robots + hiding the load more button.

classes/class-wpcom-liveblog-lazyloader.php

@@ -37,11 +37,6 @@ public static function is_enabled() {
 			 */
 			self::$enabled = (bool) apply_filters( 'liveblog_enable_lazyloader', true );
 
-			// Disable lazy loading for robots
-			if ( self::$enabled && self::is_robot() ) {
-				self::$enabled = false;
-			}
-
 			// Disable lazy loading on archived liveblogs
 			if ( 'enable' != WPCOM_Liveblog::get_liveblog_state() ) {
 				self::$enabled = false;
@@ -51,23 +46,6 @@ public static function is_enabled() {
 		return self::$enabled;
 	}
 
-	/**
-	 * Checks if the current user is a robot.
-	 *
-	 * @return bool
-	 */
-	private static function is_robot() {
-
-		// Variant determiner for caches.
-		if ( function_exists( 'vary_cache_on_function' ) ) {
-			vary_cache_on_function(
-				'return isset( $_SERVER[\'HTTP_USER_AGENT\'] ) && preg_match( \'/bot|crawl|slurp|spider/i\', $_SERVER[\'HTTP_USER_AGENT\'] );'
-			);
-		}
-
-		return isset( $_SERVER['HTTP_USER_AGENT'] ) && preg_match( '/bot|crawl|slurp|spider/i', $_SERVER['HTTP_USER_AGENT'] );
-	}
-
 	/**
 	 * Returns the number of initially displayed Liveblog entries.
 	 *

js/liveblog-lazyloader.js

@@ -10,6 +10,11 @@
 		initialize: function() {
 			lazyloader.entrySets = [];
 
+			if ( navigator.userAgent.match(/bot|crawl|slurp|spider/i) ) {
+				$( '.liveblog-load-more' ).hide();
+				return; //no lazy loading for robots
+			}
+
 			lazyloader.setBusy();
 			lazyloader.fetchEntries();