Add a macro for generating current service links

This is a convenient way of being able to link into pages of the the app
from guidance. It handles the case where the current service is not
known, either because the user hasn’t chosen one, or because they’re not
signed in.

This commit also cleans up some unused Jinja imports.

app/templates/components/current-service-link.html

@@ -0,0 +1,7 @@
+{% 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>
+  {% else %}
+    <b class="govuk-!-font-weight-bold">{{ link_text|capitalize }}</b>
+  {% endif %}
+{% endmacro %}