Make pools default to user-level storage

[junbl]

Pools currently are stored at the channel level and anyone in that channel can roll them. However, seems like most common use cases are for the user that started the pool to roll it.

Update the database to support this, add this as an option in Scope, and update scope_or_default to reflect this. Also update the docs accordingly.

Still not sure what the database should look like here - can make a separate user_pool table with the same structure, or just add the user id to the existing pool tables and update the composite unique key.