Close **Prune hook_task Table (#10741)**
Added a cron job to delete webhook deliveries in the hook_task table. It can be turned on/off and the schedule controlled globally via app.ini. The data can be deleted by either the age of the delivery which is the default or by deleting the all but the most recent deliveries _per webhook_.
Note: I had previously submitted pr #11416 but I closed it when I realized that I had deleted per repository instead of per webhook. Also, I decided allowing the settings to be overridden via the ui was overkill. Also this version allows the deletion by age which is probably what most people would want.
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