fix: prohibit manager changes on non-live pools

[rflechtner]

fixes https://github.com/KILTprotocol/ticket/issues/3808

Disallows manager changes after a pool has been moved to refunding or destroying.

Checklist:

• I have verified that the code works
  • No panics! (checked arithmetic ops, no indexing array[3] use get(3), ...)
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• I have left the code in a better state
• I have documented the changes (where applicable)
  • Either PR or Ticket to update the Docs
  • Link the PR/Ticket here