Add group_id to payload from notifications.

TNL-299

@gwprice fire away! I'm not happy about 2 spaces for indents, but that what was already there. I'm also not crazy about just looking for a substring inside of the dict, but given that I couldn't find documentation for what the expected format of the dict should be, I went with the style of the existing test.

@dan-f please also review

REQUIRED: @gwprice, @dan-f 
FYI: @andy-armstrong, @olmar 
Sorry I missed that comment. Here's the format:

```
<user_id>: {
    <course_id>: {
        <thread_id>: {
            ...
            "group_id": <group_id>
            ...
        }
    }
}
```