system hooks for group creation / deletion don't contain owners

Summary

The system hooks group_create and group_destroy don't contain owner information. The examples in the documentation still do.

Steps to reproduce

• set up system hook
• create or delete group

What is the current bug behavior?

Data sent by the hook:

"owner_name":null,
"owner_email":null

What is the expected correct behavior?

An array of owners (because there can be multiple) is sent:

"owners": [{
  "id": 41,
  "name": "John Smith",
  "username": "johnsmith",
  "email": "johnsmith@gmail.com"
}, {
  "id": 42,
  "name": "Maria Smith",
  "username": "maria"
}]

Users without a public e-mail should probably not have the email included.

Possible fixes

Use model.owners instead of model.owner in SystemHooksService