Use Visual Review Tools NPM package

Summary

Currently we have duplicate code in the https://gitlab.com/gitlab-org/visual-review-tools and in CE. We want the visual-review-tools repo to be the master, and for CE to install the NPM package as a dependency and copy it over to the assets in webpack.

Related issue: visual-review-tools#9 (closed)

Improvements

Remove duplicate code and having to do 2 MRs for every change.

Risks

Getting the NPM package to work as seamlessly as if it were still part of the normal codebase.

Involved components

https://gitlab.com/gitlab-org/gitlab-ce/tree/master/app/assets/javascripts/visual_review_toolbar