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/guidance/index.html

@@ -10,9 +10,9 @@
 
   <h1 class="heading-large">Guidance</h1>
 
-  <p>This guidance is for teams using GOV.UK Notify to send emails, text messages and letters.</p>
+  <p class="govuk-body">This guidance is for teams using GOV.UK Notify to send emails, text messages and letters.</p>
 
-  <p>It explains how to:</p>
+  <p class="govuk-body">It explains how to:</p>
 
   <ul class="list list-bullet">
     <li><a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.edit_and_format_messages') }}">edit and format messages</a></li>
@@ -23,7 +23,7 @@
 
   <h2 class="heading-medium">More information</h2>
 
-  <p>The GOV.UK Service Manual has advice on:</p>
+  <p class="govuk-body">The GOV.UK Service Manual has advice on:</p>
 
   <ul class="list list-bullet">
     <li><a class="govuk-link govuk-link--no-visited-state" href="https://www.gov.uk/service-manual/design/sending-emails-and-text-messages">planning and writing text messages and emails</a></li>