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.
21f73b29c1