Add feature graphic to be show automatically in app stores

https://gitlab.com/tom79/mastalab/issues/590

Please add a feature graphic to the fastlane tree in order to get it shown automatically in app stores such as F-Droid and Google Play.

Use the image https://cdn.mastalab.app/wp-content/uploads/2018/08/banner.png in PNG with dimensions 1024 x 500 and add it under the name featureGraphic.png in https://gitlab.com/tom79/mastalab/tree/develop/fastlane/metadata/android/en-US/images

Mastalab version: 1.31.1

Android version: 8.1.0