Investigate creating a token copy of a mutated creature

Followup from #14900 . The added logic gets the characteristics correct for all test cases in MutateTest, but there are still some peculiarities.

Incomplete list of things to consider
- [ ] The client GUI can't find an image to show for the token copy of mutate stack, need to set the top card as image for the created token
- [ ] Maybe wrong characteristics when copying a transforming double faced card, haven't specifically checked

Start with the mutation logic from #14900 and the existing MutateTest cases. Investigate how the token copy selects its image for the client GUI and how characteristics are copied from transforming double-faced cards. Done means the token displays the top card image and the copied characteristics are correct.

Written by the indexing model from the issue text.