Draft: Add alias option for GDK version of gitlab-ui

What does this MR do?

Use the GDK version of gitlab-ui for easier local development.

Depends on gitlab-development-kit!1924 (merged)

Adding gitlab_ui.enabled: true to gdk.yml adds a gitlab-ui service. This updates webpack to use this version, allowing easier local testing of gitlab-ui

Doesn't currently work for CSS, as I don't know how to check environment variables with sprockets' SCSS

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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