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/support/bat-phone.html

@@ -15,21 +15,21 @@
     )}}
     <div class="govuk-grid-row">
       <div class="govuk-grid-column-two-thirds">
-        <p>
+        <p class="govuk-body">
           First, check the
           <a class="govuk-link govuk-link--no-visited-state" href="https://status.notifications.service.gov.uk">system status page</a>.
           You do not need to contact us if
           the problem you’re having is listed on that page.
         </p>
-        <p>
+        <p class="govuk-body">
           Otherwise, contact us using the emergency email address we
           gave you or your service manager when we made your service live.
         </p>
-        <p>
+        <p class="govuk-body">
           We’ll reply within 30 minutes and give you hourly updates
           until the problem’s fixed.
         </p>
-        <p>
+        <p class="govuk-body">
           We do not offer out of hours support if your service is in
           trial mode.
         </p>