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/pricing/index.html

@@ -20,13 +20,13 @@
 
   <p>
     {% if not current_user.is_authenticated %}
-      <a href="{{ url_for('main.register') }}">Create an account</a> then add as many different services as you need to.</p>
+      <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.register') }}">Create an account</a> then add as many different services as you need to.</p>
     {% else %}
       You can add as many different services as you need to.
     {% endif %}
   </p>
 
-  <p>When you add a new service it will start in <a href="{{ url_for('main.trial_mode_new') }}">trial mode</a>.</p>
+  <p>When you add a new service it will start in <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.trial_mode_new') }}">trial mode</a>.</p>
 
   <h2 class="heading-medium" id="emails">Emails</h2>
   <p>It’s free to send emails through Notify.</p>
@@ -40,7 +40,7 @@
     <li>25,000 free text messages for other public sector services</li>
   </ul>
   <p>It costs 1.58 pence (plus VAT) for each text message you send after you've used your free allowance.</p>
-  <p>See <a href="{{ url_for('main.how_to_pay') }}">how to pay</a>.
+  <p>See <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.how_to_pay') }}">how to pay</a>.
 
   <h3 class="heading-small" id="long-text-messages">Long text messages</h3>
   <p>If a text message is longer than 160 characters (including spaces), it’ll be charged as more than one message:</p>