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/pricing/how-to-pay.html

@@ -13,7 +13,7 @@
       Invoices
     </h2>
 
-    <p>
+    <p class="govuk-body">
       We’ll send your organisation an invoice each quarter if you:
     </p>
 
@@ -22,15 +22,15 @@
       <li>send letters</li>
     </ul>
 
-    <p>
+    <p class="govuk-body">
       If your organisation has more than one service, you'll see a breakdown of each service on your invoice.
     </p>
 
-    <p>
+    <p class="govuk-body">
       If the value of an invoice is less than £250 (before <abbr title="Value Added Tax">VAT</abbr>), we’ll add it to the total for the next quarter to save time and effort.
     </p>
 
-    <p>
+    <p class="govuk-body">
       You can pay by BACS, debit card, or credit card.
     </p>
 
@@ -38,19 +38,19 @@
       Purchase orders
     </h2>
 
-    <p>
+    <p class="govuk-body">
       If your organisation’s estimated spend is more than £500 per quarter (before <abbr title="Value Added Tax">VAT</abbr>), you need to send us a purchase order (PO).
     </p>
 
-    <p>
+    <p class="govuk-body">
       Your organisation should raise a single <abbr title="purchase order">PO</abbr> for the estimated cost of all its services. You can update the PO anytime if your usage increases.
     </p>
 
-    <p>
+    <p class="govuk-body">
       You may need to set up the Cabinet Office as a supplier before you can raise a <abbr title="purchase order">PO</abbr>.
     </p>
 
-    <p>
+    <p class="govuk-body">
        <a class="govuk-link govuk-link--no-visited-state" href="{{ support_link }}">Contact us</a> if you need help raising a purchase order.
     </p>