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/manage-users/confirm-edit-user-email.html

@@ -17,11 +17,11 @@
 <div class="govuk-grid-row">
   <div class="govuk-grid-column-full">
   {% call form_wrapper() %}
-    <p>New email address:</p>
+    <p class="govuk-body">New email address:</p>
     <div class="panel panel-border-wide bottom-gutter">
-      <p>{{ new_email }}</p>
+      <p class="govuk-body">{{ new_email }}</p>
     </div>
-    <p>We will send {{ user.name }} an email to tell them about the change.</p>
+    <p class="govuk-body">We will send {{ user.name }} an email to tell them about the change.</p>
     {{ page_footer('Confirm') }}
   {% endcall %}
   </div>