Fix newlines in HTML emails

This problem was masked because the email message component was also
replacing newlines with `<br>`s.

Implements:
- [ ] https://github.com/alphagov/notifications-utils/pull/60

app/templates/components/email-message.html

@@ -50,7 +50,7 @@
       {% if not expanded %}
       <div class="email-message-body-wrapper">
       {% endif %}
-        {{ body|nl2br }}
+        {{ body }}
       {% if not expanded %}
       </div>
       <div class='toggle' tabindex='0'>...<span class='visually-hidden'>show full email</span></div>