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/verification-not-received.html

@@ -11,8 +11,8 @@
   <div class="govuk-grid-column-two-thirds">
     <h1 class="heading-large">Resend security code</h1>
 
-    <p>Text messages sometimes take a few minutes to arrive. If you do not receive a security code, Notify can send you a new one.</p>
-    <p>
+    <p class="govuk-body">Text messages sometimes take a few minutes to arrive. If you do not receive a security code, Notify can send you a new one.</p>
+    <p class="govuk-body">
       {{ govukButton({
         "element": "a",
         "text": "Resend security code",
@@ -21,7 +21,7 @@
     </p>
 
     <h2 class="heading-medium">If your mobile number has changed</h2>
-    <p>You’ll need to:</p>
+    <p class="govuk-body">You’ll need to:</p>
     <ul class="govuk-list govuk-list--bullet">
       <li>find a member of your team who has permission to manage settings, team and usage</li>
       <li>ask them to change the mobile number for your account</li>