Move the Visual Review Toolbar to its own app

What

Let's move the Visual Review App files to their very own project!

Why

We are consistently running into issues that bottom out in the value of having the toolbar separate from the main application. This will reduce confusion about folder structure and allow us to make progress on testing (#63244 (moved)), headers (#62541 (closed)), and the CDN question (#62468 (closed)). It will also make it easy to create an NPM package. (I know we can use Yarn Workspaces & Lerna, but I do believe that separate repo ➡ separate package is the simplest, most boring solution.) This will also help with concerns about needing to copy-paste on an MR-by-MR basis.

Now is a great time to do this move, because the toolbar is only slightly enmeshed with app. If we wait longer, there will be much more to unravel.

This is a followup on !28736 (merged), in particular https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28736#note_175491575.

It will help us work without maneuvering in within our big repo and make its usage more flexible. It also removes some complexity from our app.

In order for this issue to be closed:

• A new project should be created in the Gitlab organization ▶ https://gitlab.com/gitlab-org/visual-review-tools
• The files currently in Gitlab CE should be moved
• The webpack configs should be adapted to work in the new environment ▶ visual-review-tools!2 (merged)
• Add prettier, eslint visual-review-tools!7 (merged)
• We should be able to deploy the script to an appropriate CDN. Discussion at ▶ https://gitlab.com/gitlab-org/gitlab-ce/issues/62541 ▶ solution outlined in visual-review-tools#9 (closed), involves pushing to NPM
• Add in the test application and test deploy location that I am currently using to be available for all developers
• Copy over / add necessary documentation and license information
• Make project public visual-review-tools#17 (closed)