Add sass via npm to compile stylesheets

This allows us to pass variables to the design system, and will make
updating easier.

Use this to apply the design system styles globally, so that parts of
the search result page can be rendered from markdown. If we want to
restrict this more later, we can write more specific selectors that extend
the GOV.UK styles, like this:
https://github.com/ministryofjustice/cjs_scorecard_exploratory_analysis/blob/develop/cjs_test_app/scss/markdown-styles.scss

I've set up the package.json so we manually run `npm run sass` when we
want to recompile the sass. I figure this is probably good enough for us
as we won't need to change stylesheets very often.

README.md

@@ -1,14 +1,11 @@
 ## Quick start
 
-Project contains a poetry dependency file for installation with `poetry install`
-To run `poetry run python manage.py runserver`
+You will need `npm` (for javascript dependencies) and poetry (for python dependencies).
 
-## Configuration
-
-The following should be set using environment variables:
-
-- CATALOGUE_URL
-- CATALOGUE_TOKEN
+1. Run `npm install` and then `npm run sass` to compile the stylesheets.
+2. Run `poetry install` to install python dependencies
+3. Set environment variables CATALOGUE_URL and CATALOGUE_TOKEN (TODO: add dotenv support)
+4. Run `poetry run python manage.py runserver`
 
 ## Current Endpoints