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

tests/app/main/views/test_two_factor.py

@@ -17,7 +17,12 @@ def test_should_render_two_factor_page(
             'email': api_user_active.email_address}
     response = client.get(url_for('main.two_factor'))
     assert response.status_code == 200
-    assert '''We’ve sent you a text message with a security code.''' in response.get_data(as_text=True)
+    page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
+    assert page.select_one('main p').text.strip() == (
+        'We’ve sent you a text message with a security code.'
+    )
+    assert page.select_one('input')['type'] == 'tel'
+    assert page.select_one('input')['pattern'] == '[0-9]*'
 
 
 def test_should_login_user_and_should_redirect_to_next_url(