Use scheduled_task crontab field verification in
admin/tool/task to unify how Moodle deals with
crontab definition and its verification.
This helps remove duplicated code and fix
crontab definition not allowed in the web form,
but actually was valid.
Updated crontab fields precision on task_scheduled
table to have enough room for the worst case:
all possible different values separated by comma.
Centralise management of all types of cron tasks with registration, scheduling,
parallel task conflicts(blocking) and running once off tasks, all using an
administration screen.
This is a combination of several issues:
MDL-25502: Added "black magic" task allocator for cron
MDL-25503: Add step to cron to run all scheduled tasks
MDL-25504 cron: Refactor to use scheduler
MDL-25505: Add an admin interface to schedule tasks via cron.
MDL-25507: Add support for adhoc tasks.