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_test.go

@@ -0,0 +1,54 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"testing"
+)
+
+func TestCommandWebhookPreSave(t *testing.T) {
+	h := CommandWebhook{}
+	h.PreSave()
+	if len(h.Id) != 26 {
+		t.Fatal("Id should be generated")
+	}
+	if h.CreateAt == 0 {
+		t.Fatal("CreateAt should be set")
+	}
+}
+
+func TestCommandWebhookIsValid(t *testing.T) {
+	h := CommandWebhook{}
+	h.Id = NewId()
+	h.CreateAt = GetMillis()
+	h.CommandId = NewId()
+	h.UserId = NewId()
+	h.ChannelId = NewId()
+
+	for _, test := range []struct {
+		Transform     func()
+		ExpectedError string
+	}{
+		{func() {}, ""},
+		{func() { h.Id = "asd" }, "model.command_hook.id.app_error"},
+		{func() { h.CreateAt = 0 }, "model.command_hook.create_at.app_error"},
+		{func() { h.CommandId = "asd" }, "model.command_hook.command_id.app_error"},
+		{func() { h.UserId = "asd" }, "model.command_hook.user_id.app_error"},
+		{func() { h.ChannelId = "asd" }, "model.command_hook.channel_id.app_error"},
+		{func() { h.RootId = "asd" }, "model.command_hook.root_id.app_error"},
+		{func() { h.RootId = NewId() }, ""},
+		{func() { h.ParentId = "asd" }, "model.command_hook.parent_id.app_error"},
+		{func() { h.ParentId = NewId() }, ""},
+	} {
+		tmp := h
+		test.Transform()
+		err := h.IsValid()
+		if test.ExpectedError == "" && err != nil {
+			t.Fatal("hook should be valid")
+		} else if test.ExpectedError != "" && test.ExpectedError != err.Id {
+			t.Fatal("expected " + test.ExpectedError + " error")
+		}
+		h = tmp
+	}
+}