V2->V3 room migration fails if run a second time

[debuglevel]

The first room migration works if the room has shared access.
All subsequent will fail when .save() is called on the shared access object.

See also #6017

[debuglevel]

docker exec -ti greenlight3_greenlight bundle exec rails runner "Room.destroy_all" deletes all rooms. Might be useful if somebody is testing their migration and unhappy about the errors.