Add CSS linting through stylelint

Include CSS linting by default for our CSS/SCSS files, originally
introduced in cockpit. The stylelint configuration is copied from
760a4628282e02fbcef3.

.stylelintrc.json

@@ -0,0 +1,38 @@
+{
+  "extends": "stylelint-config-standard-scss",
+  "rules": {
+	"declaration-colon-newline-after": null,
+	"selector-list-comma-newline-after": null,
+
+	"at-rule-empty-line-before": null,
+	"declaration-colon-space-before": null,
+	"declaration-empty-line-before": null,
+	"custom-property-empty-line-before": null,
+	"comment-empty-line-before": null,
+	"scss/double-slash-comment-empty-line-before": null,
+	"scss/dollar-variable-colon-space-after": null,
+
+	"custom-property-pattern": null,
+	"declaration-block-no-duplicate-properties": null,
+	"declaration-block-no-redundant-longhand-properties": null,
+	"declaration-block-no-shorthand-property-overrides": null,
+	"declaration-block-single-line-max-declarations": null,
+	"font-family-no-duplicate-names": null,
+	"function-url-quotes": null,
+	"indentation": null,
+	"keyframes-name-pattern": null,
+	"max-line-length": null,
+	"no-descending-specificity": null,
+	"no-duplicate-selectors": null,
+	"scss/at-extend-no-missing-placeholder": null,
+	"scss/at-import-partial-extension": null,
+	"scss/at-mixin-pattern": null,
+	"scss/comment-no-empty": null,
+	"scss/dollar-variable-pattern": null,
+	"scss/double-slash-comment-whitespace-inside": null,
+	"scss/no-global-function-names": null,
+	"scss/operator-no-unspaced": null,
+	"selector-class-pattern": null,
+	"selector-id-pattern": null
+  }
+}