Add stats for threaded conversations

Eric Eastwoodrequested to merge
2376-add-threaded-conversation-stats into develop

Add stats for threaded conversations

Fix https://gitlab.com/gitlab-org/gitter/webapp/issues/2376

Testing strategy

  1. Ensure things didn't break (we can't really test if the stat is emitted):
  2. Enable threaded discussions in a room
  3. Send a message
  4. Send a thread message
  5. Disable threaded discussions in a room
Edited by 🤖 GitLab Bot 🤖

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