support new WhatsNew fields in index-v1 (+ l18n fixes)

At long last, this adds support for a "What's New" field, a 500 character description of what has changed in the latest release of an app. fdroidserver currently only supports a single WhatsNew field that comes from the CurrentVersionCode of the app. Google Play and Fastlane Supply support a WhatsNew field per-release, but we don't use that data anywhere, and implementing that in the data structures would add a lot of complexity since Apk would then need to have its own "localized" section like App does. That said, this current structure should not prevent the full per-APK WhatsNew support from added.

This is safe to merge before the related fdroidserver support gets merged since the Jackson parsing and the UI code handle unfilled fields just fine.