Check data consistency at import

Addresses and closes #23 (closed)

At json import, the event is exported as a dict. Then, the input field of the user input is checked to match this dict. If False, the input fails.

To make the comparison reliable I sorted all exported lists.

Note that this comparison will fail if between export and import any data is changed – even data that might not be scheduling relevant, e.g. if the name of an AK is changed.