Add stats for when someone opens the read by popover

Add stats for when someone opens the read by popover

Context: https://gitlab.com/gitlab-org/gitter/webapp/-/issues/2500/designs/Screen_Shot_2020-04-30_at_3.44.57_PM.png#note_334772490

cc @mnearents

Testing strategy

1. Send a message
2. Have another user read the message
3. Hover the read by checkmark in the top-left of the message
4. Notice the network request go out /api/private/statsc -> { stats: [{stat: "chat-item.show-read-by", count: 1}] }