feat: add POST /api/v1/installation/users to seed user

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

@@ -0,0 +1,9 @@
+import Config from '../models/config.js';
+
+export async function authorizeInstallation(request, response, next) {
+  if (await Config.isInstallationCompleted()) {
+    return response.status(403).end();
+  } else {
+    next();
+  }
+};