Make validation and remove conflicting data when copying existing event

[Rikuoja]

Currently, it is possible to copy an existing event that has e.g. a place that no longer exists or keywords that no longer exist. This will result in errors when trying to submit a new event with these values.

Instead, when copying an existing event to the form, run validation on the fields and empty any fields containing e.g. location or keywords that no longer exist.