Scheduled reminders for activities based on "created date"

Among other things, this will require adding a created_date timestamp to the `civicrm_activity` table, like so ALTER TABLE `civicrm_activity` ADD `created_date` TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP COMMENT 'Date/time of activity creation'; and then immediately setting civicrm_activity.created_date to NULL for all existing activities.

This would be useful for supporting a customizable pseudo-immediate notifications to assignees, targets, source contacts, etc. – a more flexible step up from the "Notify Activity Assignees" behavior.

This is a good idea, right? Assuming so, a PR is in the works.