Rewrite URLs in CSS based on environment

Fix for issue that caused this revert:

https://github.com/alphagov/notifications-admin/pull/3196

Note:

gulp-css-url-adjuster operates on an Abstract
Syntax Tree (AST) derived from `main.css`. The
CSS output from this loses the compression
gulp-sass applies.

This moves compression out of Sass, to a step
after the URLs are adjusted.

package.json

@@ -31,6 +31,7 @@
     "gulp-babel": "8.0.0",
     "gulp-base64-inline": "1.0.4",
     "gulp-better-rollup": "4.0.1",
+    "gulp-clean-css": "4.2.0",
     "gulp-concat": "2.6.1",
     "gulp-include": "2.3.1",
     "gulp-sass": "4.0.2",