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 
It could also be `unless current_thread.group_id.nil?`. Personally, I am ambivalent among all of these options.