Extract code to define brand background

Setting the color to `$white` or `#fff` while setting the background to
`$brand` is a pattern we were using in many places. Since we're going to
edit it in order to fix the `::selection` behavior, we're defining the
pattern in one place.

app/assets/stylesheets/mixins/colors.scss

@@ -0,0 +1,8 @@
+@mixin brand-background {
+  background-color: $brand;
+  color: $white;
+}
+
+%brand-background {
+  @include brand-background;
+}