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 
IMO, it's better to add parens/braces that are otherwise unnecessary than to use a line continuation:

```
post(
  "/api/v1/notifications",
  {
    from: CGI::escape(start_time.to_s),
    to: CGI::escape(end_time.to_s),
    user_ids: subscription.subscriber_id
  }
)
```