Fix links to current service pages from guidance

This wasn’t working because:
- macros don’t have access to `current_service` from within their scope
- there were no tests for this behaviour

app/templates/components/service-link.html

@@ -1,6 +1,6 @@
-{% macro current_service_link(endpoint, link_text) %}
-  {% if current_service %}
-    <a class="govuk-link" href="{{ url_for(endpoint, service_id=current_service.id) }}">{{ link_text }}</a>
+{% macro service_link(service, endpoint, link_text) %}
+  {% if service %}
+    <a class="govuk-link" href="{{ url_for(endpoint, service_id=service.id) }}">{{ link_text }}</a>
   {% else %}
     <b class="govuk-!-font-weight-bold">{{ link_text|capitalize }}</b>
   {% endif %}