PLT-1649: add response_url support for custom slash commands (#6739)

* add response_url support for custom slash commands

* pr suggestions

* pr update / suggestion

* test fix

model/command_webhook.go

@@ -0,0 +1,65 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"net/http"
+)
+
+type CommandWebhook struct {
+	Id        string
+	CreateAt  int64
+	CommandId string
+	UserId    string
+	ChannelId string
+	RootId    string
+	ParentId  string
+	UseCount  int
+}
+
+const (
+	COMMAND_WEBHOOK_LIFETIME = 1000 * 60 * 30
+)
+
+func (o *CommandWebhook) PreSave() {
+	if o.Id == "" {
+		o.Id = NewId()
+	}
+
+	if o.CreateAt == 0 {
+		o.CreateAt = GetMillis()
+	}
+}
+
+func (o *CommandWebhook) IsValid() *AppError {
+	if len(o.Id) != 26 {
+		return NewAppError("CommandWebhook.IsValid", "model.command_hook.id.app_error", nil, "", http.StatusBadRequest)
+	}
+
+	if o.CreateAt == 0 {
+		return NewAppError("CommandWebhook.IsValid", "model.command_hook.create_at.app_error", nil, "id="+o.Id, http.StatusBadRequest)
+	}
+
+	if len(o.CommandId) != 26 {
+		return NewAppError("CommandWebhook.IsValid", "model.command_hook.command_id.app_error", nil, "", http.StatusBadRequest)
+	}
+
+	if len(o.UserId) != 26 {
+		return NewAppError("CommandWebhook.IsValid", "model.command_hook.user_id.app_error", nil, "", http.StatusBadRequest)
+	}
+
+	if len(o.ChannelId) != 26 {
+		return NewAppError("CommandWebhook.IsValid", "model.command_hook.channel_id.app_error", nil, "", http.StatusBadRequest)
+	}
+
+	if len(o.RootId) != 0 && len(o.RootId) != 26 {
+		return NewAppError("CommandWebhook.IsValid", "model.command_hook.root_id.app_error", nil, "", http.StatusBadRequest)
+	}
+
+	if len(o.ParentId) != 0 && len(o.ParentId) != 26 {
+		return NewAppError("CommandWebhook.IsValid", "model.command_hook.parent_id.app_error", nil, "", http.StatusBadRequest)
+	}
+
+	return nil
+}