Allow API to assign new user to a specific organization (#21775)

* Allow API to assign new user to a specific organization

* Add defer block to test

* Add API tests and return 400 instead of 500 for bad orgId

* Minor test improvements

pkg/services/sqlstore/org.go

@@ -220,6 +220,18 @@ func DeleteOrg(cmd *models.DeleteOrgCommand) error {
 	})
 }
 
+func verifyExistingOrg(sess *DBSession, orgId int64) error {
+	var org models.Org
+	has, err := sess.Where("id=?", orgId).Get(&org)
+	if err != nil {
+		return err
+	}
+	if !has {
+		return models.ErrOrgNotFound
+	}
+	return nil
+}
+
 func getOrCreateOrg(sess *DBSession, orgName string) (int64, error) {
 	var org models.Org