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/check/column-errors.html

@@ -9,7 +9,7 @@
 {% set file_contents_header_id = 'file-preview' %}
 {% macro skip_to_file_contents() %}
   <p class="govuk-visually-hidden">
-    <a href="#{{ file_contents_header_id }}">Skip to file contents</a>
+    <a class="govuk-link govuk-link--no-visited-state" href="#{{ file_contents_header_id }}">Skip to file contents</a>
   </p>
 {% endmacro %}
 
@@ -150,7 +150,7 @@
         ) }}
       {% endif %}
     </div>
-    <a href="#content" class="back-to-top-link">Back to top</a>
+    <a href="#content" class="govuk-link govuk-link--no-visited-state back-to-top-link">Back to top</a>
   </div>
 
   {% if not request.args.from_test %}
@@ -176,7 +176,7 @@
               (not errors or recipients.more_rows_than_can_send) and
               displayed_index != preview_row
             ) %}
-              <a href="{{ url_for('.check_messages', service_id=current_service.id, template_id=template.id, upload_id=upload_id, row_index=displayed_index, original_file_name=original_file_name) }}">{{ displayed_index }}</a>
+              <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.check_messages', service_id=current_service.id, template_id=template.id, upload_id=upload_id, row_index=displayed_index, original_file_name=original_file_name) }}">{{ displayed_index }}</a>
             {% else %}
               {{ displayed_index }}
             {% endif %}