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.
We can use the `link` mixin. Note this mixin uses anchor-color instead
of brand-color; by default, they're both the same, so we probably meant
anchor-color here.
The `overflow: hidden` applied to the `.callout` selector made the full
width background invisible, meaning this section hasn't looked properly
on very large screens since commit 701378d02.
