Add Vue left-menu to /home/learn

Add Vue left-menu to /home/learn

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

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 http://localhost:5000/home/learn
2. Click a link on the learn page as a sanity check
3. Click a room on the left menu to go to chat room