Add Vue left-menu to community home

Add Vue left-menu to community home

Part of https://gitlab.com/gitlab-org/gitter/webapp/issues/2150

/<community>/home

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

Try the following with and without the feature toggle:

1. Visit a community home /<community>/home
2. Make sure the page is scrollable
3. Click the profile icon in the top-right to ensure JavaScript is loaded
4. Click a room on the left menu to go to chat room