Add some letter spacing to two factor textbox

Entering, or reading back sequences of digits is easier when they’re a
bit more spaced out.

This is because we read words as shapes, but read numbers
digit-by-digit.

So this commit adjusts the tracking of the type to put a bit more space
in for textboxes that are going to accept digits.

app/assets/stylesheets/components/textbox.scss

@@ -54,3 +54,8 @@
 .textbox-right-aligned {
   text-align: right;
 }
+
+.extra-tracking .form-control {
+  padding-left: 5px;
+  letter-spacing: 0.04em;
+}