Responsive display utilities (#20934)

* Explore responsive display utils, but with a twist: lowest breakpoint has no breakpoint modifier in the class name
* make floats use the same format, add float-none mixin

scss/mixins/_float.scss

@@ -4,3 +4,6 @@
 @mixin float-right {
   float: right !important;
 }
+@mixin float-none {
+  float: none !important;
+}