Reliable selection of admin user (#22509) (#22511)

Backport #22509

Co-authored-by: Sybren <122987084+drsybren@users.noreply.github.com>

models/user/user.go

@@ -1227,7 +1227,10 @@ func GetUserByOpenID(uri string) (*User, error) {
 // GetAdminUser returns the first administrator
 func GetAdminUser() (*User, error) {
 	var admin User
-	has, err := db.GetEngine(db.DefaultContext).Where("is_admin=?", true).Get(&admin)
+	has, err := db.GetEngine(db.DefaultContext).
+		Where("is_admin=?", true).
+		Asc("id"). // Reliably get the admin with the lowest ID.
+		Get(&admin)
 	if err != nil {
 		return nil, err
 	} else if !has {