Add dollar-variables to order/order Stylelint rule

This way we've been able to detect a bug where we were assigning a value
to the `$global-left` variable instead of assigning it to either the
`left` or `right` properties. This, however, was the right behavior,
since adding a `left` indentation to the link wasn't correct because its
parent element already had padding on the sides.

.stylelintrc.yml

@@ -40,6 +40,7 @@ rules:
   shorthand-property-no-redundant-values: true
   value-no-vendor-prefix: true
   order/order:
+    - dollar-variables
     - type: at-rule
       name: extend
     - type: at-rule