Added login to navigation.

- changes in wording
- added text to flight new page
Added one more image to carousel.
Added one more image to carousel.
Improved session handling of flights.

Values are preserved when navigating back from show to new.
Improved session handling of cruise.

Values are preserved when navigation back from show to new.
Enhanced session handling for fix compensations.

Preserved values when navigating back to new fix compensation.

Clean-up of activities after checkout.
Adjusted wording.
Added 2-column layout to thanks page with additional image.
Added JS to automatically calculate amount-co2 for fix compensations.
Prevent adding activities multiple times when editing them.

Separated create/edit functionality.