Refactor Get/Create Direct Channel into one function (#9867)

* refactor GetDirectChannel and CreateDirectChannel in one function

* remove CreateDirectChannel plugin api and update GetDirectChannel and GetGroupChannel plugin api

* update tests

app/webhook.go

@@ -617,7 +617,7 @@ func (a *App) HandleIncomingWebhook(hookId string, req *model.IncomingWebhookReq
 			if result := <-a.Srv.Store.User().GetByUsername(channelName[1:]); result.Err != nil {
 				return model.NewAppError("HandleIncomingWebhook", "web.incoming_webhook.user.app_error", nil, "err="+result.Err.Message, http.StatusBadRequest)
 			} else {
-				if ch, err := a.GetDirectChannel(hook.UserId, result.Data.(*model.User).Id); err != nil {
+				if ch, err := a.GetOrCreateDirectChannel(hook.UserId, result.Data.(*model.User).Id); err != nil {
 					return err
 				} else {
 					channel = ch