Add re-upload button and back link to the preview page

Re-upload button is only shown if file failed validation.

Change wording of re-upload buttons

Make test we test right buttons on letter upload preview page

Also remove double backlink

app/templates/views/check/row-errors.html

@@ -52,7 +52,7 @@
       {{ file_upload(
         form.file,
         action=url_for('.send_messages', service_id=current_service.id, template_id=template.id),
-        button_text='Re-upload your file'
+        button_text='Upload your file again'
       ) }}
     </div>
     <a href="#content" class="back-to-top-link">Back to top</a>