Create an admin_setting_configcrontab object

There are a few config settings around the place which set various types of schedules, typically legacy code from before the scheduled task api. Over time some of these ge refactored and end up with no custom schedule, and other remain with a GUI schedule which relies on the cron schedule being set to * * * * * to make it work.

So why not the best of both worlds? I'm thinking of an admin_setting_configcrontab object which edits the same config as the task api, but means that we can have a much more user friendly admin ui which is still within the same settings page as the other settings for the same topic.