Add unread indicators to menu bar items on Vue left-menu

Add unread indicators to menu bar items on Vue left-menu

Fix https://gitlab.com/gitlab-org/gitter/webapp/issues/2178

Testing strategy

1. Add the feature toggle, node ./scripts/utils/feature-toggle.js --name vue-left-menu --description "Left menu using Vue (alpha)"
2. Enable the feature toggle
   • Enable: http://localhost:5000/api_web/features/vue-left-menu/1
   • Disable: http://localhost:5000/api_web/features/vue-left-menu/0
3. Have an unread in a room, notice the "All" menu bar item has a green circle
4. Open the room and mark the messages as read. Notice the green circle go away
5. Have an mention in a room, notice the "All" menu bar item has a orange circle
6. Open the room and mark the messages as read. Notice the orange circle go away
7. Have an unread in a one to one room, notice "People' menu bar item has a green circle
8. Open the room and mark the messages as read. Notice the green circle go away