fix(comments): add an activity when there is a new comment

so we will save as part of activity who should get notified about it.
and push an activity notification for those users so they can update
their activity list.

SD-684 SD-656

@ioanpocol what do you think?
So now the user receives 2 notifications, one with the id of the item and one with the list on notified users id(s) (generated by add_activity).
Is this info enough for frontend in order to show the required message?
If there are multiple simultan comments seams to be a problem to peer the two notifications.