Be able to delete a long running search from the UI

[brendanheywood]

• [x] It doesn't matter if its has finished or not
• [x] if you delete a search then its just instantly deleted
• [x] do the action_link js confirm dialog before you delete anything
• [x] each time the running adhoc task tries to update the progress (eg every 10 seconds of 5%) it should test if the record still exists and if not then it will gracefully log it and exit the task cleanly so it is not considered a fail delay and retry
• [x] the adhoc task should clean up its temp files upon deletion