Migrate sass imports

[khawkins98]

We need to migrate from the old import style sass (https://sass-lang.com/documentation/breaking-changes/import/) but I'm no really happy with the extra verboseness;

• before:
  • border: 8px solid $mg-color-blue-600;
• after:
  • border: 8px solid variables.$mg-color-blue-600;

I've only rather lazily used the migration tool (npx sass-migrator module --migrate-deps stories/assets/scss/style.scss) so there may be a better way to handle this. It's not urgent yet, it's mostly annoying warn messages, which we may want to simply silence for the time being.

Sometimes, your dependencies have deprecation warnings that you can’t do anything about. You can silence deprecation warnings from dependencies while still printing them for your app using the --quiet-deps flag on the command line, or the quietDeps option in the JavaScript API.

In #159, I had made a basic attempt, but this change will be very disruptive, and we'll need to do this a large effort all at once.