(including spam)

... automatically when it goes from false to true

unless explicitly unsubscribed

(also if service_desk_enabled is off)

for easier adding for EE

Resolve "Deleting a user shouldn't delete associated records"

See merge request !1581

Open new board after creating it

Closes gitlab-ce#29611

See merge request !1449

1. Have `MigrateToGhostUser` be a service rather than a mixed-in module, to keep
   things explicit. Specs testing the behavior of this class are moved into a
   separate service spec file.

2. Add a `user.reported_abuse_reports` association to make the
   `migrate_abuse_reports` method more consistent with the other `migrate_`
   methods.