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/api/callbacks/delivery-status-callback.html

@@ -17,7 +17,7 @@
         back_link=url_for(back_link, service_id=current_service.id)
       ) }}
 
-      <p>
+      <p class="govuk-body">
         When you send an email or text message, we can tell you if Notify was able to deliver it.
         Read the Callbacks section of our <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.documentation') }}">API documentation</a> for more information.
       </p>

app/templates/views/api/callbacks/received-text-messages-callback.html

@@ -15,7 +15,7 @@
   ) }}
   <div class="govuk-grid-row">
     <div class="govuk-grid-column-five-sixths">
-      <p>
+      <p class="govuk-body">
         When you receive a text message in Notify, we can forward it to your system.
         Check the <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.callbacks') }}"> callback documentation </a> for more information.
       </p>