Handle default group membership

[johnstray]

A setting to determine the default group membership of users, when the user is a member of a group that no longer exists (group was deleted) or was not added to any group.

This determines the restrictiveness of permissions for users that fall into the above category.

Currently, as all relevant functions return false when they fail (ie. getGroupPermissions returns false when group file not found) the default action is to deny all permissions. A 'Deny All Permissions' option will be added to retain the current default permissiveness (ie. deny all on fail).