Add Validation to Prevent a Node from Adding itself as a Moderator

[jashot7]

Currently, a purchase will not go through if a buyer selects the seller also as the moderator of a transaction. But, it would be nice to see the server add validation to prevent the case where a node adds itself as its own moderator.

The desktop currently contains client-side prevention of a node adding itself as a moderator, but the server needs to prevent it as a possibility.