Fix "Unread notifications for child messages"

Closes #2244 (closed)

• child messages being stored with other unread messages
• unread banner opens TMF
• viewing unread messages in TMF marks them as read
• handle situation where the banner hides at the bottom of MMF even though there are unread thread messages
• include threaded messages in email notifications
• mark all child messages as read if the feature toggle is off or disable this whole feature.
  • when distributing unread messages we don't know whether the users have feature toggle off or on

The diff line count is large just because of a redesign of a chat-item test structure and resulting change in one snapshot file

Testing strategy

1. have user A and B in a room

2. let user A send 20 messages to the room, e.g. just numbers 1,2,3,4,5,6,7,8,9,0,1,2...

3. then start a thread on a few of those messages

4. let user B enter the room with small viewport (so all 20 messages won't fit on 1 screen)

   1. validate you see both top and bottom unread banner (top with all messages above the fold and bottom with the number of thread messages)
   2. clicking the top banner takes you to the first unread message
   3. clicking the bottom banner takes you down to the first unread bellow the fold
   4. when reading through all MMF messages, the bottom banner still shows the number of unread thread messages
   5. clicking the banner should open the TMF with the oldest thread message and then every next click opens the next TMF
   6. viewing the thread message in TMF marks it as read

5. Try to add a few mentions here and there. Note that they always take precedence over unreads

6. the child messages are automatically included in the unread notification email

   • User A and B are in a room. User A creates a thread message
   • Add the Mandrill webapp prod secrets to your local webapp .env file
     • Don't forget to re-export the .env into your updated environment, https://gitlab.com/gitlab-org/gitter/webapp#configure-service-secrets
     • You can skip this step if you want to use the fake mailer
   • wait 1 minute and then run the cron task ./scripts/crons/send-unread-notifications.js
   • User B should have email with the thread message in their mailbox