Restructure webhook module (#22256)

Previously, there was an `import services/webhooks` inside
`modules/notification/webhook`.
This import was removed (after fighting against many import cycles).
Additionally, `modules/notification/webhook` was moved to
`modules/webhook`,
and a few structs/constants were extracted from `models/webhooks` to
`modules/webhook`.

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/notification/notification.go

@@ -16,7 +16,6 @@ import (
 	"code.gitea.io/gitea/modules/notification/mail"
 	"code.gitea.io/gitea/modules/notification/mirror"
 	"code.gitea.io/gitea/modules/notification/ui"
-	"code.gitea.io/gitea/modules/notification/webhook"
 	"code.gitea.io/gitea/modules/repository"
 	"code.gitea.io/gitea/modules/setting"
 )
@@ -36,7 +35,6 @@ func NewContext() {
 		RegisterNotifier(mail.NewNotifier())
 	}
 	RegisterNotifier(indexer.NewNotifier())
-	RegisterNotifier(webhook.NewNotifier())
 	RegisterNotifier(action.NewNotifier())
 	RegisterNotifier(mirror.NewNotifier())
 }