Concurrent access

[bohnje]

Currently we don't have any protection against concurrent access to the same obligation (if one person is editing it as another deletes it)

I doubt we will fix this before the final snapshot though

[KyleBrownCS]

The user isn't able to save any edits if the obligation has been deleted. In the future the error message can be improved to tell the user that the entry was removed and unable to be saved. Our server still runs properly so this only inconveniences the user (arguably bad).