Use tabular numbers for all big numbers

Reimplements https://github.com/alphagov/notifications-admin/pull/169

> 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.

— 5f38012f94/docs/mixins.md (tabular-numbers)

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

@@ -4,7 +4,7 @@
   display: block;
 
   &-number {
-    @include bold-48;
+    @include bold-48($tabular-numbers: true);
     display: block;
   }
 
@@ -21,7 +21,7 @@
   @extend %big-number;
 
   .big-number-number {
-    @include bold-36();
+    @include bold-36($tabular-numbers: true);
   }
 
 }
@@ -31,7 +31,7 @@
   @extend %big-number;
 
   .big-number-number {
-    @include bold-24();
+    @include bold-24($tabular-numbers: true);
   }
 
 }