Publish application.css within npm package

In order to avoid pulling application.css directly from gitlab-ce/ee twice (within gitlab-ui and design.gitlab.com), I propose we do it once here, then publish it within our npm package as a testing fixture of sorts.

This would have two benefits:

• Importing application.css within design.gitlab.com will be much simpler. We can simply @import '../node_modules/.....' instead of using curl to pull down some artifact from a pipeline in another repo. design.gitlab.com won't have to be "aware" of this process or of the gitlab-ce/ee repos at all.
• Now, when rendering design.gitlab.com we can be sure that the application.css styles we're seeing/using there are exactly the same ones which the version of gitlab-ui we're using was tested against.

/cc @ClemMakesApps