Better flight cancellation/new flight handling

[István Váradi]

• It should be possible to start a new flight when the previous one has finished
• It should be possible to cancel the flight on every page (with confirmation, of course, and the button should be a bit detached from the others).

In case of a new flight or a cancelled flight, we should go back to the flight selection page without logging in again, and the list of flights should be refreshed.

Perhaps add also a refresh button on the flight list page.