Add restrictions to replace and create series

The minimum number of public keys is now three and the number of
required signatures must be less than the number of public keys

This fixes task #1112
I thought about this and thought it weird to exclude the admittedly likely unusual case of x of x voting pools. But the task explicitly said strict less than, so I went with that. I guess I should have challenged that when I had the chance. But we definitely need to make sure the tasks are accurately defined in the future.