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/signedout.html

@@ -28,7 +28,7 @@
           <h1>
             Send emails, text messages and letters to your users
           </h1>
-          <p>
+          <p class="govuk-body">
             Try GOV.UK Notify now if you work in central government, a&nbsp;local authority, or the NHS.
           </p>
           <div class="button-container">
@@ -52,7 +52,7 @@
         <h2>
           Control your content
         </h2>
-        <p>
+        <p class="govuk-body">
           You do not need any technical knowledge to create email,
           text&nbsp;message or letter templates.
         </p>
@@ -68,7 +68,7 @@
         <h2>
           See how your messages perform
         </h2>
-        <p>
+        <p class="govuk-body">
           Track how many messages you’ve sent and
           find out which ones are not being delivered.
         </p>
@@ -87,7 +87,7 @@
     </h2>
     <div class="govuk-grid-row">
       <div class="govuk-grid-column-one-half">
-        <p>
+        <p class="govuk-body">
           Upload a spreadsheet of email&nbsp;addresses or
           phone&nbsp;numbers and Notify sends the messages.
         </p>
@@ -97,7 +97,7 @@
         >
       </div>
       <div class="govuk-grid-column-one-half">
-        <p>
+        <p class="govuk-body">
           Integrate the GOV.UK Notify API with your web&nbsp;application or
           back office system.
         </p>
@@ -133,7 +133,7 @@
           organisations
         </div>
       </div>
-      <p>
+      <p class="govuk-body">
         See the
         <a class="govuk-link govuk-link--no-visited-state" href="https://www.gov.uk/performance/govuk-notify/government-services">list of services and organisations</a>.
       </p>
@@ -163,7 +163,7 @@
           <p class="align-with-big-number-hint">
             There’s no monthly charge, no setup fee and no&nbsp;procurement process.
           </p>
-          <p>Find out more about <a class="govuk-link govuk-link--no-visited-state" href="https://www.notifications.service.gov.uk/pricing">pricing</a>.
+          <p class="govuk-body">Find out more about <a class="govuk-link govuk-link--no-visited-state" href="https://www.notifications.service.gov.uk/pricing">pricing</a>.
           </p>
         </div>
       </div>
@@ -175,11 +175,11 @@
         <h2 class="with-keyline">
           The team
         </h2>
-        <p>
+        <p class="govuk-body">
           GOV.UK Notify is built by the Government Digital Service
           and is supported 24 hours a day, 7 days a week.
         </p>
-        <p>
+        <p class="govuk-body">
           <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.support') }}">Contact us</a> if you have a question or want
           to give feedback.
         </p>