Introduce Vue into the codebase

Introduce Vue into the codebase behind a feature flag

Part of https://gitlab.com/gitlab-org/gitter/webapp/issues/1987

Code split from https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1360 to make a smaller MR to review/merge

• Introduces Vue into the codebase
  • Uses a left-menu component but it isn't any sort of left-menu at this point (just a red box)
• Includes server-side rendering. Based off of https://ssr.vuejs.org/guide/
• Using scoped LESS styles
• Using VueX

Feature flag

node ./scripts/utils/feature-toggle.js --name vue-left-menu --description "Left menu using Vue (alpha)"

• Enable: http://localhost:5000/api_web/features/vue-left-menu/1
• Disable: http://localhost:5000/api_web/features/vue-left-menu/0

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Closes #1987 (closed)