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/error/404.html

@@ -6,13 +6,13 @@
         <h1 class="heading-large">
           Page not found
         </h1>
-        <p>
+        <p class="govuk-body">
           If you typed the web address, check it is correct.
         </p>
-        <p>
+        <p class="govuk-body">
           If you pasted the web address, check you copied the entire address.
         </p>
-        <p>
+        <p class="govuk-body">
           If the web address is correct or you selected a link or button, <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.support') }}">contact us</a>.
         </p>
       </div>