Fix "Chat privately" in user popover not changing left-menu room highlight

Fix "Chat privately" in user popover not changing left-menu room highlight

Fix https://gitlab.com/gitlab-org/gitter/webapp/issues/2229

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. Use the "Chat privately" button in a user popover to open the one to one room with them
4. Notice the left-menu highlight has changed to that room