Allow textboxes to have width set, default to 2/3

For entering 2 or 3fa codes, we want a textbox that’s just over 6 characters
wide.

To do this, a width can now be passed to the textbox macro. The possible widths
are the same as those provided by GOV.UK Elements, and in the same format (eg
1-4, 1-2, 2-3…)

This commit also adds a new width (5em) which is suitable for 3fa codes, and
adds it to the verify page.

app/templates/views/verify.html

@@ -17,11 +17,13 @@ GOV.UK Notify | Confirm email address and mobile number
     <form autocomplete="off" method="post">
       {{ textbox(
         form.email_code,
+        width='5em',
         help_link=url_for('.check_and_resend_email_code'),
         help_link_text='I haven’t received an email'
       ) }}
       {{ textbox(
         form.sms_code,
+        width='5em',
         help_link=url_for('.check_and_resend_text_code'),
         help_link_text='I haven’t received a text'
       ) }}