Enable numeric keypad for text message code

If you’re signing in on a phone, it’s easier to type the two factor code
with a numeric keypad. The most reliable way to get the numeric keypad
to show up on multiple devices is:
- `type='tel'` (not `type='number'` because that’s only meant for
  numbers, not string of digits, ie `01234` is not a number)
- `pattern='[0-9]*'`, without which it doesn’t work on iOS

Based on the guidance here:
- https://github.com/alphagov/govuk-design-system-backlog/issues/74
- https://docs.google.com/document/d/1wozIhOdt6wvlgqVReauUnlsJI-3fqUlNuQFwUI7tqAA/edit

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" class="extra-tracking">
+    <form autocomplete="off" method="post" class="extra-tracking" novalidate>
       {{ textbox(
         form.sms_code,
         width='5em',