Add dark-theme support to Vue left-menu

Add dark-theme support to Vue left-menu

Fix https://gitlab.com/gitlab-org/gitter/webapp/issues/2184

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 (doesn't work for NLI anonymous users though)
   • Enable: http://localhost:5000/api_web/features/vue-left-menu/1
   • Disable: http://localhost:5000/api_web/features/vue-left-menu/0
3. Toggle on the dark-theme on and notice the left-menu change to dark
4. Toggle off the dark-theme off and notice the left-menu to light