Migrate DB: rename namespace, automigrate user

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

db.go

@@ -187,7 +187,9 @@ func (h *Headscale) initDB() error {
 		return err
 	}
 
-	err = db.AutoMigrate(&Namespace{})
+	_ = db.Migrator().RenameTable("Namespace", "User")
+
+	err = db.AutoMigrate(&User{})
 	if err != nil {
 		return err
 	}