Use Vue.js for the left-menu (iframe barrier)

Start using Vue.js for the left-menu,

• Introduces Vue.js into the codebase which GitLab is already using, https://docs.gitlab.com/ee/development/fe_guide/
  • Upgrade from Backbone/Marrionette
• The other goal is to remove the app/chat iframe barrier and start on the journey towards single-page application (SPA)

Things to keep in mind

• Single top-level VueX state that drives the application
• Convert gitter-realtime-client Backbone live-collection to VueX store
• Server-side rendering
• Full page reloads for things like the userhome, explore page, and community home

...

• Vue left menu v1, https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1360
  • Introducing Vue.js into the codebase, https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1396
  • Move client-side context to module, https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1398
  • Refactor chat-internal to use async/await, https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1401
  • Update gitter-realtime-client@2.0.0 to include updates to sorts for Vue left menu, https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1403
  • https://gitlab.com/gitlab-org/gitter/realtime-client/merge_requests/23
  • Add Jest for Vue testing, https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1404