EES-2938 Remove node-sass and pin sass at 1.32.13

This change removes `node-sass` in favour of `sass` (the Dart
implementation) so that we should no longer get node-gyp errors in the
build pipeline.

Note that we have pinned `sass` at 1.32.13, instead of the latest
version due to `govuk-frontend` using deprecated features. This was
unfortunately causing too much unnecessary noise in the frontend dev
server logs. This should hopefully be resolved by `govuk-frontend` at
some point the future. See: alphagov/govuk-frontend#2238