There is an automatic data privacy request created to get rid of all the
personal data upon the user deletion. With the actual user record
absent, the data privacy requests page throws an error. Let's do here
same as we do in `delete_incomplete_users_task` and let us not delete
the actual user record.
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.
