The `GroupManager` constructor ensures the structure of the `GroupList`
table, and if the table is newly-created, then some initial data is
seeded.
We previously used the names `default` to represent the group that
accounts are defaultly registered to, and `guest` to represent players
that have not logged in yet. We now instead refer to the group names
specified by the config (`DefaultRegistrationGroupName` and
`DefaultGuestGroupName` respectively)
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