Revamp background utilities and add new .text-white utility

- Rather than mix multiple properties in our color utilities, this splits all color and all background utils into separate classes.
- Adds new .text-white class to help lighten text color for darker backgrounds

scss/mixins/_background-variant.scss

@@ -2,7 +2,6 @@
 
 @mixin bg-variant($parent, $color) {
   #{$parent} {
-    color: #fff !important;
     background-color: $color !important;
   }
   a#{$parent} {

scss/utilities/_text.scss

@@ -32,6 +32,10 @@
 
 // Contextual colors
 
+.text-white {
+  color: #fff !important;
+}
+
 @include text-emphasis-variant('.text-muted', $text-muted);
 
 @include text-emphasis-variant('.text-primary', $brand-primary);