Add Vue left menu to all pages

Add Vue left menu to all pages,

• /home
  • https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1360
• /home/explore
  • https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1413
• /home/learn
  • https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1422
• /community/home
  • https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1421

Currently mobile views are rendered separately. https://gitlab.com/gitlab-org/gitter/webapp/issues/2151 tracks removing these separate mobile views and instead always use the same mobile optimized page