Feature: Add a is_callsign_editable flag to Booking

[daveroverts]

Situation: Flights were imported containing a call_sign, but no aircraft_type. According to BookingsImport, when both call_sign and aircraft_type are filled in, these details are not allowed to be edited.

However, because aircraft_type was missing, is_editable is set to true. This meant 2 things:

• Whenever somebody doesn't confirm / cancels their booking, both callsign and acType where set to null.
• The callsign could be changed

The following needs to be done:

• Add is_callsign_editable flag to Booking
• If able without breaking too much: Rename is_editable to is_actype_editable
• Ensure BookingsImport can handle the new flag
• Change views + BookingController so it can handle the new flag as needed