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/new-password.html

@@ -15,7 +15,7 @@
       <h1 class="heading-large">
         Create a new password
       </h1>
-      <p>
+      <p class="govuk-body">
         You can now create a new password for your account.
       </p>
       {% call form_wrapper() %}
@@ -23,7 +23,7 @@
         {{ page_footer("Continue") }}
       {% endcall %}
     {% else %}
-      <p>
+      <p class="govuk-body">
         Message about email address does not exist. Some one needs to figure out the words here.
       </p>
     {% endif %}