Threaded conversations
Threaded conversations are common in many modern chat applications and are very helpful for busy rooms where multiple conversations may be occurring at the same time.
Main issue in the webapp: https://gitlab.com/gitlab-org/gitter/webapp/issues/741
### Proposal
Support one level of threading.


### Use threaded conversations now
All of our current work on threaded conversations is hidden under a feature toggle.
You can enable threaded conversations for a room that you are an admin of. Go to room settings and check the `Enable threaded conversations.` checkbox. After that, you can `Start a thread` from the context menu on each message.
epic