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/main/forms.py

@@ -160,8 +160,15 @@ def password(label='Password'):
                                      Blacklist(message='Choose a password that’s harder to guess')])
 
 
+class SMSCode(StringField):
+    def __call__(self, **kwargs):
+        return super().__call__(
+            type='tel', pattern='[0-9]*', **kwargs
+        )
+
+
 def sms_code():
-    return StringField('Text message code', validators=[
+    return SMSCode('Text message code', validators=[
         DataRequired(message='Can’t be empty'),
         Regexp(regex='^\d+$', message='Numbers only'),
         Length(min=5, message='Not enough numbers'),