Use ‘Continue’ not ‘Next’ for button text

The service manual recommends to:

> Make sure your ‘Continue’ button is:
>
> - labelled ‘Continue’, not ‘Next’
> - aligned to the left so users don’t miss it

– https://www.gov.uk/service-manual/design/question-pages

app/templates/views/send-test.html

@@ -29,7 +29,7 @@
         </div>
       {% endif %}
     </div>
-    {{ page_footer('Next', back_link=back_link) }}
+    {{ page_footer('Continue', back_link=back_link) }}
   </form>
 
   {{ template|string }}