Do not allow removal of decision mentioning deleted member

[tomudding]

Deleting a decision that references a deleted member is something that has never happened and will hopefully never happen. However, we might want to add some code that deals with this.

There are several ways to go about this:

1. If it is the last reference, delete the deleted member. If it is not the last reference, nothing happens to the deleted member.
2. Do not allow deletion of these kind of decisions.

I personally favour the second option, if necessary you can always destroy a decision.

This has a very low priority.