Forced Group Ownership

[QuickMythril]

The UPDATE_GROUP tx allows for someone to change the owner of a group to another address, without requiring any confirmation from that address. One very simple solution to this problem would just be to require that a New Owner already be a member of the group. Currently if the new owner is not a member, they will be automatically added. Instead the UPDATE_GROUP tx should simply be rejected as invalid when the New Owner is not already in the group.