Development Branch: DB/Cache Cleanup

[nyanginator]

I suppose this is more of a feature request than an issue, but on a site where many courses/modules/users are frequently created/updated/deleted, the history DB table and Moodle Cache may fill up rather quickly with obsolete/unnecessary/unwanted records.

It would be nice to have options to:

• Delete clean URL history from DB and from Moodle Cache prior to a certain date.
  
  
• Selectively delete specific URLs from DB and from Moodle Cache.
  
  
• Automatically delete URL history from DB and from Moodle Cache for deleted courses/modules/users. It appears that courses/modules aren't cleared from the DB until their Scheduled Tasks are run (i.e. through cron). Perhaps URL history should also use cron to do cleanup, probably also using the Scheduled Tasks API?
  
  
• Have a Purge Caches button on the settings page to purge caches for just clean URLs, instead of having to do Purge All Caches to refresh. It would be similar to how the Theme Selector allows you to just Clear Theme Caches.