Move webhook to a standalone package under modules (#8747)

* Move webhook to a standalone package under modules

* fix test

* fix comments

modules/webhook/main_test.go

@@ -0,0 +1,16 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package webhook
+
+import (
+	"path/filepath"
+	"testing"
+
+	"code.gitea.io/gitea/models"
+)
+
+func TestMain(m *testing.M) {
+	models.MainTest(m, filepath.Join("..", ".."))
+}