Refactor sms_code functionality into the class

So it’s all in one place, not two.

tests/app/main/views/test_two_factor.py

@@ -21,6 +21,9 @@ def test_should_render_two_factor_page(
     assert page.select_one('main p').text.strip() == (
         'We’ve sent you a text message with a security code.'
     )
+    assert page.select_one('label').text.strip(
+        'Text message code'
+    )
     assert page.select_one('input')['type'] == 'tel'
     assert page.select_one('input')['pattern'] == '[0-9]*'