Child messages only show in TMF

Closes #2202 (closed)

Last outstanding task

• child messages are marking their parent as unread message

*Note: unread notifications are handled in https://gitlab.com/gitlab-org/gitter/webapp/issues/2244

What's been done

• child messages not showing in the Main Message Feed (MMF)
• sending child message now adds temporary message to the TMF and then replaces it with stored message
• failing to send child message results in red highlight on the child message

Success	Failure

Testing strategy

1. Login as 2 users in 2 different tabs
2. both of those users open the same room and start a thread on the same message
3. submitt message as one user and see that the other user received live update to the TMF
4. child messages are showing in TMF and not in MMF

• node ./scripts/utils/feature-toggle.js --name threaded-conversations
• http://localhost:5000/api_web/features/threaded-conversations/1