Add properties-alphabetical-order Stylelint rule

We hadn't added this rule before because there was no such rule in
scss-lint. Instead, we were following it without a linter, and so we
unintentionally broke it sometimes.

But now we're using Stylelint, so we can add the rule and let the linter
check we're still following it.

.stylelintrc.yml

@@ -51,6 +51,7 @@ rules:
       name: include
       hasBlock: true
     - rules
+  order/properties-alphabetical-order: true
   scss/at-else-closing-brace-space-after: always-intermediate
   scss/at-else-empty-line-before: never
   scss/at-extend-no-missing-placeholder: true