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

app/command_help.go

@@ -0,0 +1,44 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import (
+	"github.com/mattermost/platform/model"
+	"github.com/mattermost/platform/utils"
+	goi18n "github.com/nicksnyder/go-i18n/i18n"
+)
+
+type HelpProvider struct {
+}
+
+const (
+	CMD_HELP = "help"
+)
+
+func init() {
+	RegisterCommandProvider(&HelpProvider{})
+}
+
+func (h *HelpProvider) GetTrigger() string {
+	return CMD_HELP
+}
+
+func (h *HelpProvider) GetCommand(T goi18n.TranslateFunc) *model.Command {
+	return &model.Command{
+		Trigger:          CMD_HELP,
+		AutoComplete:     true,
+		AutoCompleteDesc: T("api.command_help.desc"),
+		DisplayName:      T("api.command_help.name"),
+	}
+}
+
+func (h *HelpProvider) DoCommand(args *model.CommandArgs, message string) *model.CommandResponse {
+	helpLink := *utils.Cfg.SupportSettings.HelpLink
+
+	if helpLink == "" {
+		helpLink = model.SUPPORT_SETTINGS_DEFAULT_HELP_LINK
+	}
+
+	return &model.CommandResponse{GotoLocation: helpLink}
+}