dashboards: add validation to delete dashboard by slug

Validates that there are only one folder/dashboard having that slug,
otherwise returns 412 Precondition Failed

pkg/services/sqlstore/dashboard.go

@@ -386,3 +386,14 @@ func GetDashboardSlugById(query *m.GetDashboardSlugByIdQuery) error {
 	query.Result = slug.Slug
 	return nil
 }
+
+func GetDashboardsBySlug(query *m.GetDashboardsBySlugQuery) error {
+	var dashboards = make([]*m.Dashboard, 0)
+
+	if err := x.Where("org_id=? AND slug=?", query.OrgId, query.Slug).Find(&dashboards); err != nil {
+		return err
+	}
+
+	query.Result = dashboards
+	return nil
+}