Add indexer job to clean out geshi fragment cache

This addresses FS#1418 & FS#923

https://bugs.dokuwiki.org/index.php?do=details&task_id=1418
https://bugs.dokuwiki.org/index.php?do=details&task_id=923

I've put config value to 28 days.  To be meaningful it should be higher than ```$conf['cachetime']```.  Geshi output is included within the renderers .xhtml cache.  The Geshi fragments go alongside the instruction cache speeding up rendering.