Add order-last grid class

Created by: cahbb

This PR adds a new .order-last grid class.

It does the exact opposite thing of .order-first. It reorders the target element to be the last using order: $columns + 1;

<div class="container">
  <div class="row">
    <div class="col order-sm-last">
      First by default, but last on sm
    </div>
    <div class="col">
      Second by default, and still second on sm
    </div>
    <div class="col order-sm-first">
      Last by default, but first on sm
    </div>
  </div>
</div>

This gives the user a more intuitive way of understanding how the columns are ordered. Saying "I want this to be the last child on sm" and not begin to figure out how many columns you have in your grid and so on. The child is going to be what you say - the last child.

As it is now you would use the class .order-sm-12 instead of .order-sm-last in the above example.

docs/4.0/layout/grid.md

@@ -534,14 +534,14 @@ Use `.order-` classes for controlling the **visual order** of your content. Thes
 {% endexample %}
 </div>
 
-There's also a responsive `.order-first` class that quickly changes the order of one element by applying `order: -1`. This class can also be intermixed with the numbered `.order-*` classes as needed.
+There are also responsive `.order-first` and `.order-last` classes that change the `order` of an element by applying `order: -1` and `order: 13` (`order: $columns + 1`), respectively. These classes can also be intermixed with the numbered `.order-*` classes as needed.
 
 <div class="bd-example-row">
 {% example html %}
 <div class="container">
   <div class="row">
-    <div class="col">
-      First, but unordered
+    <div class="col order-last">
+      First, but last
     </div>
     <div class="col">
       Second, but unordered

scss/mixins/_grid-framework.scss

@@ -46,14 +46,12 @@
         }
       }
 
-      .order#{$infix}-first {
-        order: -1;
-      }
+      .order#{$infix}-first { order: -1; }
+
+      .order#{$infix}-last { order: $columns + 1; }
 
       @for $i from 0 through $columns {
-        .order#{$infix}-#{$i} {
-          order: $i;
-        }
+        .order#{$infix}-#{$i} { order: $i; }
       }
 
       // `$columns - 1` because offsetting by the width of an entire row isn't possible