Import base scss vars from gitlab-ui

What does this MR do?

Most of the color, size, and space vars are defined in gitlab-ui, then duplicated in variables.scss. This allows us to remove any duplicate values when making updates such as #213570 (closed)

Downside means that code search for these variables in gitlab won't tell you their hex values. So maybe we could either leave them there and simply comment out and say "go to this gitlab-ui file to see the hex values"