Switch CSS minifier to cssnano (#9260)

This results in around 4kB saving in CSS size and also resolves with a
particular issue on git bash which had issues with the format of the
lessc arguments.

Makefile

@@ -469,9 +469,9 @@ css: node-check $(CSS_DEST)
 
 $(CSS_DEST): node_modules $(CSS_SOURCES)
 	npx stylelint web_src/less
-	npx lessc --clean-css="--s0 -b" web_src/less/index.less public/css/index.css
-	$(foreach file, $(filter-out web_src/less/themes/_base.less, $(wildcard web_src/less/themes/*)),npx lessc --clean-css="--s0 -b" web_src/less/themes/$(notdir $(file)) > public/css/theme-$(notdir $(call strip-suffix,$(file))).css;)
-	npx postcss --use autoprefixer --no-map --replace public/css/*
+	npx lessc web_src/less/index.less public/css/index.css
+	$(foreach file, $(filter-out web_src/less/themes/_base.less, $(wildcard web_src/less/themes/*)),npx lessc web_src/less/themes/$(notdir $(file)) > public/css/theme-$(notdir $(call strip-suffix,$(file))).css;)
+	npx postcss --use autoprefixer --use cssnano --no-map --replace public/css/*
 
 .PHONY: javascripts
 javascripts:

package.json

@@ -13,11 +13,11 @@
     "babel-loader": "8.0.6",
     "core-js": "3.4.1",
     "css-loader": "3.2.0",
+    "cssnano": "4.1.10",
     "eslint": "6.6.0",
     "eslint-config-airbnb-base": "14.0.0",
     "eslint-plugin-import": "2.18.2",
     "less": "3.10.3",
-    "less-plugin-clean-css": "1.5.1",
     "postcss-cli": "6.1.3",
     "style-loader": "1.0.0",
     "stylelint": "11.1.1",