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

* Move webhook to a standalone package under modules

* fix test

* fix comments

Author: lunny

go.mod

@@ -69,6 +69,8 @@ require (
 	github.com/mattn/go-sqlite3 v1.11.0
 	github.com/mcuadros/go-version v0.0.0-20190308113854-92cdf37c5b75
 	github.com/microcosm-cc/bluemonday v0.0.0-20161012083705-f77f16ffc87a
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
+	github.com/modern-go/reflect2 v1.0.1 // indirect
 	github.com/mschoch/smat v0.0.0-20160514031455-90eadee771ae // indirect
 	github.com/msteinert/pam v0.0.0-20151204160544-02ccfbfaf0cc
 	github.com/nfnt/resize v0.0.0-20160724205520-891127d8d1b5