Allow loading a flight directly on the login page

[István Váradi]

This should be useful for offline flights. Pilots can save their flights while logged in and then fly later offline. Some other parts of the code need to take this into account:

• the online gate and ACARS systems should not work regardless of the setting
• the Finish page should default to "Offline flight"
• the Cancel flight button should not reload the list of flights, and perhaps it can go back to the login page

Perhaps the login page should not contain a file loading button, but it should have a button for "Offline" mode, which would go directly to the flight selection page.