Discuss how we handle records with deleted users in the workspaces domain

Description

Spin off from this thread !179604 (comment 2322858289). We are trying to set a precedent on how we handle records that hold references to users who can be deleted. The current options are:

• cascade delete (delete the record that the user is associated with)
• nullify the user in a column of associated records
• use the ghost user as a placeholder

All are viable options depending on the context, but we currently opt into the nullify approach in handling cluster agent mappings. We should be able to come up with a guided path to determine what option to use in any given context, to be consistent and set a precedent.

Acceptance criteria

• Product aligned guidelines on picking an approach on handling deleted users

Implementation plan