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/uploads/contact-list/column-errors.html

@@ -29,7 +29,7 @@
         <h1 class='banner-title' data-module="track-error" data-error-type="Too many rows" data-error-label="{{ upload_id }}">
           Your file has too many rows
         </h1>
-        <p>
+        <p class="govuk-body">
           Notify can store files up to
           {{ "{:,}".format(recipients.max_rows) }} rows in size. Your
           file has {{ "{:,}".format(recipients|length) }} rows.
@@ -42,7 +42,7 @@
           {{ 'this' if recipients|length == 1 else 'these' }}
           {{ recipient_count_label(recipients|length, recipients.template_type) }}
         </h1>
-        <p>
+        <p class="govuk-body">
           In <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.trial_mode_new') }}">trial mode</a> you can only
           send to yourself and members of your team
         </p>