Add a loading indicator for transitions + retry failed (GET) requests

See https://twitter.com/bjschrijver/status/1691719841508462629?s=20

During Devoxx BE ticket sales, we experience a huge load of requests.
This slowed down the whole reservation process, and it was not clear for people if the had actually managed to reserve tickets or not.

- [ ] We should add a loading indicator while we transition the reservation from one status to another
- [ ] We need also to retry failed (GET) requests when loading the following step