Fix #10850: Update Gravatar URL format to meet current standard

Gravatar's official URL syntax for loading an image is different to the
older mechanism MantisBT is still using. The official syntax is
available at:

http://en.gravatar.com/site/implement/images/

core/user_api.php

@@ -826,7 +826,7 @@ function user_get_avatar( $p_user_id, $p_size = 80 ) {
 			$t_gravatar_domain = 'https://secure.gravatar.com/';
 		}
 
-		$t_avatar_url = $t_gravatar_domain . 'avatar.php?gravatar_id=' . md5( $t_email ) . '&default=' . urlencode( $t_default_image ) . '&size=' . $t_size . '&rating=G';
+		$t_avatar_url = $t_gravatar_domain . 'avatar/' . md5( $t_email ) . '&default=' . urlencode( $t_default_image ) . '&size=' . $t_size . '&rating=G';
 		$t_result = array(
 			$t_avatar_url,
 			$t_size,