portico_error_pages: Collect zerver error pages into a folder.

Tested `link_does_not_exist.html`, `404.html` and `unsupported_browser`
pages render correctly.

zerver/views/push_notifications.py

@@ -184,7 +184,11 @@ def self_hosting_auth_redirect_endpoint(
     except ResourceNotFoundError:
         return render(request, "404.html", status=404)
     except RemoteRealmServerMismatchError:
-        return render(request, "zerver/remote_realm_server_mismatch_error.html", status=403)
+        return render(
+            request,
+            "zerver/portico_error_pages/remote_realm_server_mismatch_error.html",
+            status=403,
+        )
 
     return HttpResponseRedirect(redirect_url)