[MM-10718] Move custom branding to TE (#8871)

* move custom branding to TE
* move brand's enterprise code to server and remove BrandInterface

einterfaces/brand.go

@@ -1,15 +0,0 @@
-// Copyright (c) 2016-present Mattermost, Inc. All Rights Reserved.
-// See License.txt for license information.
-
-package einterfaces
-
-import (
-	"mime/multipart"
-
-	"github.com/mattermost/mattermost-server/model"
-)
-
-type BrandInterface interface {
-	SaveBrandImage(*multipart.FileHeader) *model.AppError
-	GetBrandImage() ([]byte, *model.AppError)
-}