Moving slash-commands out of app package (#14979)

* Moving slash-commands out of app package

* Fixing golint checks

* Fixing golangci-lint

* Fixing golangci-lint errors

app/slashcommands/command_offline.go

@@ -0,0 +1,40 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package slashcommands
+
+import (
+	goi18n "github.com/mattermost/go-i18n/i18n"
+	"github.com/mattermost/mattermost-server/v5/app"
+	"github.com/mattermost/mattermost-server/v5/model"
+)
+
+type OfflineProvider struct {
+}
+
+const (
+	CMD_OFFLINE = "offline"
+)
+
+func init() {
+	app.RegisterCommandProvider(&OfflineProvider{})
+}
+
+func (me *OfflineProvider) GetTrigger() string {
+	return CMD_OFFLINE
+}
+
+func (me *OfflineProvider) GetCommand(a *app.App, T goi18n.TranslateFunc) *model.Command {
+	return &model.Command{
+		Trigger:          CMD_OFFLINE,
+		AutoComplete:     true,
+		AutoCompleteDesc: T("api.command_offline.desc"),
+		DisplayName:      T("api.command_offline.name"),
+	}
+}
+
+func (me *OfflineProvider) DoCommand(a *app.App, args *model.CommandArgs, message string) *model.CommandResponse {
+	a.SetStatusOffline(args.UserId, true)
+
+	return &model.CommandResponse{ResponseType: model.COMMAND_RESPONSE_TYPE_EPHEMERAL, Text: args.T("api.command_offline.success")}
+}