Now that the `notify_push` module declaratively installs _its_ app [1], I should declaratively install _all_ apps. [1]: https://github.com/NixOS/nixpkgs/pull/451501
This needs to be configured through the "Nextcloud Office" app, specifically the WOPI setting is important for security (I put both the external IP, as well as `::1` and `127.0.0.1`).
