Use file-loader for sprite icons within icon.vue

What does this MR do?

Replaces use of gon.sprite_icons with an import statement as though this asset were being handled by webpack rather than sprockets. In the background, we are overriding this import statement to resolve to gon.sprite_icons anyway via our webpack config.

This is done so that icon.vue can be moved to gitlab-ui and remain agnostic about the use of gon to pass asset paths from sprockets to webpack.

Part of gitlab-ui#98 (closed)