Add Vue left-menu to /home/explore

Add Vue left-menu to /home/explore

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

Testing

There won't be any suggestions in your dev environment(suggestions come from Neo4j)

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:

Signed in

1. Visit http://localhost:5000/home/explore/ and notice "Suggested" selected
2. Select a tag notice the new tag selected
3. Custom tag via URL, http://localhost:5000/home/explore/tags/asdf and notice the tag is selected
4. Try visiting http://localhost:5000/explore/tags/asdf version and notice the redirect to http://localhost:5000/home/explore/tags/asdf

Signed out

1. Visit http://localhost:5000/explore and notice "Suggested" selected
2. Select a tag notice the new tag selected
3. Custom tag via URL, http://localhost:5000/explore/tags/asdf