add newsletter sign up to chapter pages

Author: Kimberley Cook

app/views/chapter/show.html.haml

@@ -54,3 +54,6 @@
 .py-4.py-lg-5.bg-light
   .container
     = render partial: 'shared/donation_platforms', locals: { title: t('homepage.donation_platforms.title'), with_button: true }
+
+.container.py-4.py-lg-5
+  = render partial: 'shared/newsletter'