api: allow extra recipients for pr/commit comments api methods

- adds some sort of CC functionality for notification
- allows sending emails with notifications to people who are not commit messages, or pr reviewers e.g project manager etc.

rhodecode/model/notification.py

@@ -111,6 +111,7 @@ class NotificationModel(BaseModel):
 
         # add mentioned users into recipients
         final_recipients = set(recipients_objs).union(mention_recipients)
+
         notification = Notification.create(
             created_by=created_by_obj, subject=notification_subject,
             body=notification_body, recipients=final_recipients,