refactor(installation): improve allow installation guard

packages/backend/src/helpers/allow-installation.js

@@ -0,0 +1,16 @@
+import Config from '../models/config.js';
+import User from '../models/user.js';
+
+export async function allowInstallation(request, response, next) {
+  if (await Config.isInstallationCompleted()) {
+    return response.status(403).end();
+  }
+
+  const hasAnyUsers = await User.query().resultSize() > 0;
+
+  if (hasAnyUsers) {
+    return response.status(403).end();
+  }
+
+  next();
+};