Block the use of !important in our CSS

!important can never be overridden, which is almost never what you
want. Better is to provide either a more specific selector, or
potentially a CSS @layer.

For more information, see
https://developer.mozilla.org/en-US/docs/Web/CSS/important#best_practices

.stylelintrc

@@ -6,6 +6,7 @@
     "stylelint-scss"
   ],
   "rules": {
+    "declaration-no-important": true,
     "selector-class-pattern": [
       "^[a-z][a-zA-Z0-9]+$",
       {