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/message-status.html

@@ -10,9 +10,9 @@
   <h1 class="heading-large">Delivery status</h1>
 
   <p>Notify’s real-time dashboard lets you check the status of any message, to see when it was delivered.</p>
-  <p>For <a href="{{ url_for("main.security") }}">security</a>, this information is only available for 7 days after a message has been sent. You can download a report, including a list of sent messages, for your own records.</p>
+  <p>For <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for("main.security") }}">security</a>, this information is only available for 7 days after a message has been sent. You can download a report, including a list of sent messages, for your own records.</p>
   <p>This page describes the statuses you’ll see when you’re signed in to Notify.</p>
-  <p>If you’re using the Notify API, read our <a href="{{ url_for('.documentation') }}">documentation</a> for a list of API statuses.<p>
+  <p>If you’re using the Notify API, read our <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.documentation') }}">documentation</a> for a list of API statuses.<p>
 
   <h2 id="email-statuses" class="heading-medium">Emails</h2>
   <div class="bottom-gutter-3-2">