Use a tabbed interface.

Move all current UI elements except the login screen
into the tabbed interface in `main.fxml`.

Refactor the booking controller minimally to support the
new interface and continue to use it.

Remove the following views and controllers.

- `bookings.fxml`.
- `error.fxml` and `ErrorController.java`.
- `hut_status.fxml` and `HutStatusController.java`.
- `menu.fxml` and `MenuController.java`.

Remove scene control interface from *App*.

Remove dependency on *App* from *BookingController*.