Fix I18n::MissingTranslationData exception for checkbox 'none'

This has been a problem since #2005, but it only came to light after a
production deployment.

Checkbox questions have an implicit 'none' response if the user doesn't select
any of the checkboxes. `CheckboxQuestionPresenter#response_labels` translates
the options for display in the 'Previous answers' section at the bottom of the
page.

When the translation fallbacks were removed in #2005, a
`I18n::MissingTranslationData` was being raised when the user did not select
any checkboxes.

Unfortunately, this commit [1] added a translation for the 'none' option even
though the `CheckboxSampleFlow` [2] didn't have an explicit `:none` option.
Note also that the `assert_page_has_content` was changed to check for 'None'
instead of 'none'.

In this commit, I've removed the translation for the 'none' option which gave
me a failing test which replicated the problem we'd seen in production.

I've fixed the failing test by adding an `if/else` statement to
`CheckboxQuestionPresenter#response_labels` which effectively reintroduces
the translation fallback in this very specific case.

I'm not totally happy with this solution, but hopefully it'll do for now.

[1]: f50ec82
[2]: https://github.com/alphagov/smart-answers/blob/master/test/fixtures/smart_answer_flows/checkbox-sample.rb

app/presenters/checkbox_question_presenter.rb

@@ -1,7 +1,11 @@
 class CheckboxQuestionPresenter < QuestionPresenter
   def response_labels(values)
     values.split(',').map do |value|
-      translate_option(value)
+      if value == SmartAnswer::Question::Checkbox::NONE_OPTION
+        value.to_s
+      else
+        translate_option(value)
+      end
     end
   end
 

test/fixtures/smart_answer_flows/locales/en/checkbox-sample.yml

@@ -8,4 +8,3 @@ en-GB:
           peppers: "Peppers"
           ice_cream: "Ice Cream!!!"
           pepperoni: "Pepperoni"
-          none: "None"

test/integration/engine/checkbox_questions_test.rb

@@ -59,7 +59,7 @@ class CheckboxQuestionsTest < EngineIntegrationTest
           within 'td.previous-question-title' do
             assert_page_has_content "What do you want on your pizza?"
           end
-          within('td.previous-question-body') { assert_page_has_content "None" }
+          within('td.previous-question-body') { assert_page_has_content "none" }
           within('.link-right') { assert page.has_link?("Change", href: "/checkbox-sample/y?previous_response=none") }
         end
       end