Seed initial group database with `default` and `guest` names from config

[drunderscore]

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)