[feat] enable invite codes for default new installs

- Resolves: #8474

To disable invite codes and open up to the public, it suffices to change
in your json the "invite": to false and deploy again. To generate invite
codes see https://leap.se/en/docs/platform/services/webapp#invite-codes

provider_base/templates/webapp.json

+{
+  "webapp": {
+    "invite": true
+  }
+}