- controller golem15.user.usergroups behind golem15.users.access_groups
- list with users_count from the list's own query; form with the checkbox
permission editor stored in user_groups.permissions
- creating a group with a privileged code, changing a code to or from one and
deleting a privileged group need golem15.users.manage_privileged_groups
- a group delete removes its users_groups rows in the same transaction
- UserGroup gains Fillable, Rules and the read-only UsersCount; PermissionSet.Text
- TestAdminGroups and the D-06 matrix in TestAdminPrivilegedGroups
- groups relation field on the user form, the only writer of users_groups
- UsersGroup pivot model
- AdminRelationLocks: adding or removing a group whose code is in
golem15.user.privileged_groups needs golem15.users.manage_privileged_groups,
on create and on update; refused saves answer 403 and write nothing
- TestAdminUserGroupsField and TestAdminPrivilegedGroups
