Update all links to use GOVUK Frontend style

Includes:
- turning off :visited styles to match existing
  design
- swapping heading classes used to make links bold
  for the GOVUK Frontend bold override class
- adding visually hidden text to some links to
  make them work when isolated from their context

We may need to revisit whether some links, such as
those for documentation and features, may benefit
from having some indication that their target has
been visited.

app/templates/views/documentation.html

@@ -19,12 +19,12 @@
       ('python', 'Python'),
       ('ruby', 'Ruby'),
     ] %}
-      <li><a href="https://docs.notifications.service.gov.uk/{{ key }}.html" target="_blank" rel="noopener">{{ label }}</a></li>
+      <li><a class="govuk-link govuk-link--no-visited-state" href="https://docs.notifications.service.gov.uk/{{ key }}.html" target="_blank" rel="noopener">{{ label }}</a></li>
     {% endfor %}
   </ul>
     <p>A developer should be able to set up the API client and start sending test messages in around 30 minutes. A full integration can take a few days, depending on the other systems you’re using.</p>
   <h2 class="heading-medium">Integrate directly with the API</h2>
     <p>If you cannot use the client libraries, you can integrate directly with the API instead.</p>
-<p>Read the <a href="https://docs.notifications.service.gov.uk/rest-api.html" target="_blank" rel="noopener">REST API documentation</a> (this link opens in a new tab).</p>
+<p>Read the <a class="govuk-link govuk-link--no-visited-state" href="https://docs.notifications.service.gov.uk/rest-api.html" target="_blank" rel="noopener">REST API documentation</a> (this link opens in a new tab).</p>
 
 {% endblock %}