fix(17): WR-01 keep a menu entry's subtree and parent in its menu

- The admin refuses a parent from another menu than the one the form
  names (errors.parent_menu) instead of moving the entry to the parent's
  menu without a word; BeforeSave records the named menu
  (MenuEntry.RequestedMenuID).
- An entry moved to another menu moves its descendants in the same
  transaction, so the public tree no longer drops them.

models/menu_entry.go

@@ -48,6 +48,10 @@ type MenuEntry struct {
 	IsPublished *bool      `gorm:"column:is_published"`
 	CreatedAt   *time.Time `gorm:"column:created_at"`
 	UpdatedAt   *time.Time `gorm:"column:updated_at"`
+
+	// requestedMenuID is the menu a save asked for when BeforeSave replaced
+	// it with the parent's menu (see RequestedMenuID).
+	requestedMenuID uint
 }
 
 func (MenuEntry) TableName() string { return "golem15_menu_menu_entries" }
@@ -71,7 +75,10 @@ func (MenuEntry) Rules() map[string]string {
 // an entry with a parent takes the parent's menu, and an entry without a
 // sort_order goes after the menu's last one. A missing title is stored empty
 // (dividers have none) and a missing published flag is the PHP default true.
+// When the entry named another menu than its parent's, RequestedMenuID
+// reports the menu it named; the admin refuses such a save.
 func (e *MenuEntry) BeforeSave(tx *gorm.DB) error {
+	e.requestedMenuID = 0
 	zero := func(p **int) {
 		if *p == nil {
 			v := 0
@@ -96,6 +103,9 @@ func (e *MenuEntry) BeforeSave(tx *gorm.DB) error {
 			return err
 		}
 		if len(menuIDs) > 0 && menuIDs[0] != 0 {
+			if e.MenuID != 0 && e.MenuID != menuIDs[0] {
+				e.requestedMenuID = e.MenuID
+			}
 			e.MenuID = menuIDs[0]
 		}
 	}
@@ -113,6 +123,11 @@ func (e *MenuEntry) BeforeSave(tx *gorm.DB) error {
 	return nil
 }
 
+// RequestedMenuID is the menu the last save of e named when BeforeSave
+// replaced it with the parent's menu: the parent belongs to another menu.
+// It is 0 when the entry named no menu or its parent's.
+func (e *MenuEntry) RequestedMenuID() uint { return e.requestedMenuID }
+
 // Text is the value of an optional text column, "" when null.
 func Text(s *string) string {
 	if s == nil {