flags.jinja: Don't output style="None" if there is no style parameter
topbar.jinja: Merged duplicate code into _language_item() macro
flights: Change active_page to 'flights' for "Change Aircraft" page
controllers/flight: Remove unused user parameter from return values
controllers/flight: Pass the flight.id to the select_aircraft_form

This fixes a bug where users were not able to change their aircraft
information anymore. I am a bit unsure how that bug was introduced...