add language constant for all languagues.

Please ALWAYS add language constants to all files (using addlang.sh i.e.)

lang/UTF-8/serendipity_lang_it.inc.php

@@ -971,3 +971,5 @@
 @define('PENDING_COMMENT_SUBSCRIPTION', 'Pending confirmation');
 @define('NO_COMMENT_SUBSCRIPTION', 'Not subscribed');
 @define('SUMMARY', 'Summary');
+@define('ARCHIVE_SORT_STABLE', 'Stable Archives');
+@define('ARCHIVE_SORT_STABLE_DESC', 'Sort the archive-pages descending, so they are stable and search-crawler do not have to reindex them.');