Modify helper to_displaying_string() to process NEW! notifiers

app/helpers/application_helper.rb
- Add process_new_notifier() to be called from to_displaying_string().
app/views/projects/_form.html.erb
- Display usage for NEW! notifier.
config/locales/ja.yml
- Add "message.project.usage_for_new_notifier" under "helpers".
public/stylesheets/style.css
- Add 'span.new'.