moved tab generation to PHP, no empty tabs SPR-799

I still don't like the code directly in the template there. I think it
should be moved to a helper class.

The TOC CSS seems to be broken when there are no tabs right of the TOC.

lang/en/lang.php

@@ -27,6 +27,8 @@ $lang['meta_box_jira_tickets_none'] = 'jira associated tickets found: none';
 $lang['meta_box_tags_none']         = 'tags found: none';
 $lang['js']['meta_box_toc_none']    = 'no Table of Contents available';
 
+$lang['tab_tags']                   = 'Tags';
+
 $lang['quality_trigger']            = 'toggle page analysis';
 
 $lang['prefix_tasks']               = 'open tasks: ';