Reduce utils.Cfg references (#7650)

* app.UpdateConfig method

* test fix

* another test fix

* the config override option as-was is just error prone, remove it for now

* derp

app/file.go

@@ -244,7 +244,7 @@ func GeneratePublicLinkHash(fileId, salt string) string {
 }
 
 func (a *App) UploadFiles(teamId string, channelId string, userId string, fileHeaders []*multipart.FileHeader, clientIds []string) (*model.FileUploadResponse, *model.AppError) {
-	if len(*utils.Cfg.FileSettings.DriverName) == 0 {
+	if len(*a.Config().FileSettings.DriverName) == 0 {
 		return nil, model.NewAppError("uploadFile", "api.file.upload_file.storage.app_error", nil, "", http.StatusNotImplemented)
 	}