Don’t allow paragraphs without class attribute

All paragraphs should have class="govuk-body", or be otherwise
custom-styled. This commit adds some extra checks to our test fixture
that looks for paragraphs that don’t have any styling. Our test coverage
is pretty good, so this should check almost all pages, and prevent
regressions.

I’ve done this in such a way that it can be extended for other elements
(e.g. links) in the future.

app/templates/views/notifications/notification.html

@@ -15,7 +15,7 @@
       message_count_label(1, template.template_type, suffix='') | capitalize,
       back_link=back_link
     ) }}
-    <p>
+    <p class="govuk-body">
       {% if is_precompiled_letter %}
         {% if created_by %}
           Uploaded
@@ -58,19 +58,19 @@
       {% else %}
         {% if sent_with_test_key %}
           {% if is_precompiled_letter %}
-            <p>
+            <p class="govuk-body">
               This letter passed our checks, but we will not print it because you used a test key.
             </p>
           {% else %}
-            <p>
+            <p class="govuk-body">
               We will not print this letter because you used a test key.
             </p>
           {% endif %}
         {% else %}
-          <p>
+          <p class="govuk-body">
             {{ letter_print_day }}
           </p>
-          <p>
+          <p class="govuk-body">
             Estimated delivery date: {{ estimated_letter_delivery_date|format_day_of_week }} {{ estimated_letter_delivery_date|format_date_short }}
           </p>
         {% endif %}
@@ -103,7 +103,7 @@
     {% endif %}
 
     {% if current_user.has_permissions('send_messages') and current_user.has_permissions('view_activity') and template.template_type == 'sms' and can_receive_inbound %}
-      <p>
+      <p class="govuk-body">
         <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.conversation', service_id=current_service.id, notification_id=notification_id, _anchor='n{}'.format(notification_id)) }}">See all text messages sent to this phone number</a>
       </p>
     {% endif %}