Uses class method to get list of instance admins

Re-writing this query over and over is a bad approach

bookwyrm/views/landing/about.py

@@ -19,9 +19,7 @@ def about(request):
         "status_count": models.Status.objects.filter(
             user__local=True, deleted=False
         ).count(),
-        "admins": models.User.objects.filter(
-            groups__name__in=["admin", "moderator"]
-        ).distinct(),
+        "admins": models.User.admins(),
         "version": settings.VERSION,
     }