Add mobile support to Vue left-menu

Add mobile support to Vue left-menu

Fix https://gitlab.com/gitlab-org/gitter/webapp/issues/2151

Also adds:

• When the left-menu isn't pinned,
  • Clicking on a non-active item will expand the left-menu and show that new state
  • Clicking on the same active item will toggle the left-menu

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 visiting on mobile(you can use Chrome's device toolbar) with and without the feature toggle:

• /home
• /home/learn
• /home/explore
• /community/home