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/service-settings/set-sms.html

@@ -16,16 +16,16 @@
         'Send text messages',
         back_link=url_for('main.service_settings', service_id=current_service.id)
       ) }}
-      <p>
+      <p class="govuk-body">
         You have a free allowance of
         {{ '{:,}'.format(current_service.free_sms_fragment_limit) }} text messages each
         financial year.
       </p>
-      <p>
+      <p class="govuk-body">
         It costs 1.58 pence (plus VAT) for each text message you send
         after your free allowance.
       </p>
-      <p>
+      <p class="govuk-body">
         See <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for(".pricing", _anchor="letters") }}">pricing</a> for more details.
       </p>
       {% call form_wrapper() %}