PLT-7818 Updates to post type (#7579)

* Updates to post type

* Update tests

app/webhook.go

@@ -131,6 +131,11 @@ func (a *App) CreateWebhookPost(userId string, channel *model.Channel, text, ove
 	post := &model.Post{UserId: userId, ChannelId: channel.Id, Message: text, Type: postType}
 	post.AddProp("from_webhook", "true")
 
+	if strings.HasPrefix(post.Type, model.POST_SYSTEM_MESSAGE_PREFIX) {
+		err := model.NewAppError("CreateWebhookPost", "api.context.invalid_param.app_error", map[string]interface{}{"Name": "post.type"}, "", http.StatusBadRequest)
+		return nil, err
+	}
+
 	if metrics := a.Metrics; metrics != nil {
 		metrics.IncrementWebhookPost()
 	}