Add `parentId` to threaded messages

Closes #2201 (closed)

This MR can't be merged till https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1490#note_195261493 is resolved, but apart from this blocker, it is ready.

Testing strategy

1. both vue-left-menu and threaded-conversations feature toggles are on
2. open thread message feed and submit a message through it
3. observe that the new message has correctly filled parentId
   • can be either done by looking the message up in the database
   • or it can be done by looking at the network tab in Chrome dev tools
     • select WS for WebSocket traffic
     • find subscription to /chatMessages channel
     • see the parent id in .ext.snapshot.items
     •