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/support/bat-phone.html

@@ -15,7 +15,7 @@
       <div class="column-two-thirds">
         <p>
           First, check the
-          <a href="https://status.notifications.service.gov.uk">system status page</a>.
+          <a class="govuk-link govuk-link--no-visited-state" href="https://status.notifications.service.gov.uk">system status page</a>.
           You do not need to contact us if
           the problem you’re having is listed on that page.
         </p>
@@ -33,11 +33,11 @@
         </p>
         <h2 class="heading-medium">Any other problems</h2>
         <p class="bottom-gutter-2">
-          <a href="{{ url_for('main.feedback', ticket_type='report-problem', severe='no') }}">Fill in this form</a>
+          <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.feedback', ticket_type='report-problem', severe='no') }}">Fill in this form</a>
           and we’ll get back to you by the next working day.
         </p>
         <p>
-          <a href="{{ url_for('.support') }}">Back to support</a>
+          <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.support') }}">Back to support</a>
         </p>
       </div>
     </div>

app/templates/views/support/index.html

@@ -20,7 +20,7 @@
       {{ page_footer('Continue') }}
     {% endcall %}
 
-    <p>You can also <a href="https://ukgovernmentdigital.slack.com/messages/C0E1ADVPC">contact us on Slack</a>.</p>
+    <p>You can also <a class="govuk-link govuk-link--no-visited-state" href="https://ukgovernmentdigital.slack.com/messages/C0E1ADVPC">contact us on Slack</a>.</p>
     
     <h2 class="heading-medium">Office hours</h2>
     <p>Our office hours are 9:30am to 5:30pm, Monday to Friday.</p>

app/templates/views/support/report-problem.html

@@ -19,7 +19,7 @@
       <div class="column-two-thirds">
         <div class="panel panel-border-wide">
           <p>
-            Check our <a href="https://status.notifications.service.gov.uk">system status</a>
+            Check our <a class="govuk-link govuk-link--no-visited-state" href="https://status.notifications.service.gov.uk">system status</a>
             page to see if there are any known issues with GOV.UK Notify.
           </p>
         </div>