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/platform-admin/_base_template.html

@@ -27,7 +27,7 @@
           ('Clear cache', url_for('main.clear_cache')),
         ] %}
           <li>
-            <a href="{{ url }}">
+            <a class="govuk-link govuk-link--no-visited-state" href="{{ url }}">
               {{ link_text }}
             </a>
           </li>

app/templates/views/platform-admin/reports.html

@@ -11,17 +11,17 @@
   </h1>
 
 <p>
-  <a target="_blank" href="{{ url_for('main.live_services_csv') }}">Download live services csv report</a>
+  <a class="govuk-link govuk-link--no-visited-state" target="_blank" href="{{ url_for('main.live_services_csv') }}">Download live services csv report</a>
 </p>
 
 <p>
-  <a target="_blank" href="{{ url_for('main.performance_platform_xlsx') }}">Download performance platform report (.xlsx)</a>
+  <a class="govuk-link govuk-link--no-visited-state" target="_blank" href="{{ url_for('main.performance_platform_xlsx') }}">Download performance platform report (.xlsx)</a>
 <p>
 
 <p>
-  <a href="{{ url_for('main.notifications_sent_by_service') }}">Monthly notification statuses for live services</a>
+  <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.notifications_sent_by_service') }}">Monthly notification statuses for live services</a>
 <p>
 <p>
-  <a href="{{ url_for('main.usage_for_all_services') }}">Usage for all services</a>
+  <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.usage_for_all_services') }}">Usage for all services</a>
 </p>
 {% endblock %}

app/templates/views/platform-admin/services.html

@@ -49,7 +49,7 @@
 
         {% call row() %}
           {% call field(border=False) %}
-            <a href="{{ url_for('main.service_dashboard', service_id=service['id']) }}" class="browse-list-link">{{ service['name'] }}</a>
+            <a href="{{ url_for('main.service_dashboard', service_id=service['id']) }}" class="govuk-link govuk-link--no-visited-state browse-list-link">{{ service['name'] }}</a>
           {% endcall %}
 
           {{ stats_fields('email', service['stats']) }}