Changes in custom forms render the formdata edit form unusable

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This is what happens:

1. A reservation with data is stored
2. The formdata forms are changed
3. The formdata edit form can no longer load the data

This is not an issue we can work around. We will instead show a warning in the formdata edit form.

Since the display of the formdata still works (no knowledge of the data is necessary there), the user can then copy the missing values from the manage view to the edit form by hand.

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It would be nice to show the data that could not be loaded and to make sure it is kept around after saving.