Add ESLint
## What does this MR do?
- create `package.json`
- add ESLint dependencies to `package.json`
- add JavaScript linting to CI
- add Rake task `lint:javascript`as alias for `eslint` (which itself is an alias for `npm run eslint`)
## Are there points in the code the reviewer needs to double check?
Probably not.
## Why was this MR needed?
My hovercraft was full of eels.
## What are the relevant issue numbers?
- https://gitlab.com/gitlab-org/gitlab-ce/issues/13224#note_12537431
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5006#note_13255658
See merge request !5445
Rather than constructing & then deconstructing, we know just hide & show
the sidebar. This is done so we dont' have a memory leak on the frontend
with objects getting created.
