Link project_bot user deletion audit event to its resource if possible

What does this MR do and why?

Related to #488166 (closed)

Currently, when deleting user with any user type, GitLab creates user_destroyed audit event. That audit event is linked to User. Meaning that only instance admins can see that audit event.

The goal of this MR to allow resource(group/project) owners see user_destroyed audit events that are related to the resource project_bot user deletion by linking that audit event to the resource if possible.

That MR also

• improves user_destroyed audit event creation by mentioning reason of the user deletion in the message
• ensures user_destroyed audit event is created even without current_user
  • I think we and users could benefit from having audit events about user deletions without current_user(when user is deleted from rails console for instance). See !167021 (comment 2137711286)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create project access token or group access token. (Under the hood it creates a user with project_bot type)
2. Revoke the token or simulate token expiration.
3. Visit the group/project's /audit_events page. You should see the event about the project_bot deletion with the reason in the message.

app/services/users/destroy_service.rb

@@ -126,8 +126,33 @@ def resource(user, options)
       options[:resource].presence || user.resource_bot_resource
     end
 
-    def log_bot_user_delete_audit_event(user, resource, reason_for_deletion)
-      # defined in EE
+    def reason_for_deletion(reason)
+      reason = "Reasons unknown" unless reason.present?
+
+      reason
+    end
+
+    def log_bot_user_delete_audit_event(user, resource, reason)
+      queue_message = "user will queued for deletion within next five minutes."
+      message_str = "Bot User #{user.username} scheduled for deletion, #{queue_message}"
+      audit_context = {
+        name: "bot_user_scheduled_for_deletion",
+        author: current_user,
+        scope: resource,
+        target: user,
+        target_details: user.full_path,
+        message: "#{message_str} Reason: #{reason_for_deletion(reason)}.",
+        additional_details: {}
+      }
+
+      if current_user.nil?
+        audit_context[:author] = ::Gitlab::Audit::UnauthenticatedAuthor.new(name: '(System)')
+        audit_context[:message] = message_str
+        audit_context[:additional_details][:reason] = reason_for_deletion(reason)
+        audit_context[:additional_details][:system_event] = true
+      end
+
+      ::Gitlab::Audit::Auditor.audit(audit_context)
     end
 
     def update_lag_gauge