Use tablular numbers for ‘big number’ pattern

> Tabular numbers have numerals of a standard fixed width. As all numbers have
> the same width, sets of numbers may be more easily compared. We recommend
> using them where different numbers are likely to be compared, or where
> different numbers should line up with each other, eg in tables.

The big number pattern is good candidate for tabluar numbers, especially if
we ever have these numbers update dynamically (in that case tabular numbers
won’t jump around like lining ones would).

app/assets/stylesheets/components/big-number.scss

@@ -1,6 +1,6 @@
 .big-number {
 
-  @include bold-48;
+  @include bold-48($tabular-numbers: true);
 
   &-label {
     @include core-19;

app/templates/views/styleguide.html

@@ -62,7 +62,12 @@
       {{ big_number("567") }}
     </div>
     <div class="column-one-third">
-      {{ big_number("2", "Messages delivered") }}
+      {{ big_number("£2.19", "Final cost") }}
+    </div>
+  </div>
+  <div class="grid-row">
+    <div class="column-one-third">
+      {{ big_number("711", "Queued") }}
     </div>
   </div>