https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1482

- [x] remove code that puts messages from initial render to Vuex store
- [x] display parent message when opening thread message feed
- [x] support KaTeX styles from Vue less loader
- [x] update messageMap when changing room
- [x] update messageMap when live-loading new messages / updating messages
- [x] update messageMap with messages fetched during scrolling
- [x] update messageMap when submitting/updating a new message
- [x] make the `thread-message-feed>chat-item` style more compact
- [x] format sent date and show it
- [x] handle trimming of user and message info in small space
- [x] dark theme