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.
After moving to Dart Sass, we were getting warnings like:
```
Deprecation Warning: $saturation: Passing a number without unit % (78)
is deprecated
Deprecation Warning: $lightness: Passing a number without unit % (93) is
deprecated
```
So we're passing percentages to the hsla function instead of passing
numbers without units.
