feat(admin): add GitHub repo link to admin user-links bar

Overrides the admin userlinks block in base_site.html to surface a
link to the project's GitHub repo alongside View site / Change
password / Log out.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jonfroehlich

website/templates/admin/base_site.html

@@ -14,6 +14,11 @@
 </div>
 {% endblock %}
 
+{% block userlinks %}
+  <a href="https://github.com/makeabilitylab/makeabilitylabwebsite" target="_blank" rel="noopener">GitHub</a> /
+  {{ block.super }}
+{% endblock %}
+
 {% block extrastyle %}
 {{ block.super }}
 <style>