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/templates/views/two-factor.html

@@ -15,7 +15,7 @@
 
     <p>We’ve sent you a text message with a security code.</p>
 
-    <form autocomplete="off" method="post">
+    <form autocomplete="off" method="post" class="extra-tracking">
       {{ textbox(
         form.sms_code,
         width='5em',