Add /help slash command (#6650)

* Add /help slash command

* Fix copyright year

* go fmt

* Add help command test at api4

* Fix jasonblais's issue

* Update test code

* Fix unit test issue - keep config unchanged

api4/command_help_test.go

@@ -0,0 +1,33 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package api4
+
+import (
+	"github.com/mattermost/platform/model"
+	"github.com/mattermost/platform/utils"
+	"testing"
+)
+
+func TestHelpCommand(t *testing.T) {
+	th := Setup().InitBasic()
+	Client := th.Client
+	channel := th.BasicChannel
+
+	HelpLink := *utils.Cfg.SupportSettings.HelpLink
+	defer func() {
+		*utils.Cfg.SupportSettings.HelpLink = HelpLink
+	}()
+
+	*utils.Cfg.SupportSettings.HelpLink = ""
+	rs1, _ := Client.ExecuteCommand(channel.Id, "/help ")
+	if rs1.GotoLocation != model.SUPPORT_SETTINGS_DEFAULT_HELP_LINK {
+		t.Fatal("failed to default help link")
+	}
+
+	*utils.Cfg.SupportSettings.HelpLink = "https://docs.mattermost.com/guides/user.html"
+	rs2, _ := Client.ExecuteCommand(channel.Id, "/help ")
+	if rs2.GotoLocation != "https://docs.mattermost.com/guides/user.html" {
+		t.Fatal("failed to help link")
+	}
+}