Manage Play Store assets with Fastlane

[nicolas-raoul]

Currently we store market assets in https://github.com/commons-app/marketing and manually copy to Play Store via its web UI.

Problem: It is hard to keep things in sync.

Fastlane can feed to the Play Store API the content a specially crafted folder of assets and metadata. This could solve our problem.

Ideally it would be great if we can keep the repositories separated, as assets are voluminous and not of interest to developers.