MDL-70230 admin: add setting sched. task status

MDL-70230 admin: add setting sched. task status

When defining settings that are used by scheduled tasks,
it is also useful, or even needed, to know the status
of that scheduled task to have the whole big picture of
that part of the system.

Based on the admin_setting_description, this new setting
reports its name, its status, a link to the configuration.

When adding a new setting of this type, the user can add
an extra description field to complete the whole meaning.