Multi user capability
Try to avoid that changes get overwritten when users are logged in simultaneously.
Display an error message if an entry was modified since the update form was opened.
1) Only the main entities are being checked on update (source, event, actor, place, reference. It could still be possible that e.g. a type name gets accidentally overwritten.
2) For data consistency it was decided against providing an option to overwrite data which has changed in the meantime.
3) Some advanced form elements doesn't behave like expected after an overwrite error e.g. jstree with multiple option isn't prefilled with chosen options, which shouldn't be a big issue, since the entry cant be saved in this state anyway.