feat: manage a user's groups from the user form, privileged groups locked

- 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

models/user/fields.yaml

@@ -35,6 +35,12 @@ fields:
         span: right
         tab: golem15.user::lang.user.account
         context: [create, update]
+    groups:
+        label: golem15.user::lang.user.groups
+        type: relation
+        span: full
+        emptyOption: golem15.user::lang.user.empty_groups
+        tab: golem15.user::lang.user.account
     organisation:
         label: golem15.user::lang.organisation.label
         type: relation

models/users_group.go

@@ -0,0 +1,17 @@
+package models
+
+// UsersGroup is one row of users_groups, the pivot behind User.Groups: the
+// user is a member of the group. The table has exactly these two columns and
+// a composite primary key over them. The admin user form's groups field is
+// its only writer, apart from the cleanup of a permanently deleted user or
+// group.
+type UsersGroup struct {
+	UserID      uint `gorm:"column:user_id;primaryKey"`
+	UserGroupID uint `gorm:"column:user_group_id;primaryKey"`
+}
+
+func (UsersGroup) TableName() string { return "users_groups" }
+
+func init() {
+	Register(UsersGroup{})
+}