fix: Change power level without changing memory

[krille-chan]

The problem here is that we
have not created a deep copy
of the power level map. By using .copy() we create a
deep copy now.

Closes https://github.com/famedly/product-management/issues/2412

[td-famedly]

lg

[nico-famedly]

Strictly speaking I guess there could be an edge case if the users map is set to a list, where we won't set the permissions ever now. I think those rooms probably fail to federate and so this is fine and we don't need to care about it :D