update: include "What's New" texts when they are available

update: include "What's New" texts when they are available

This uses the "What's New" entry for the CurrentVersionCode and includes it
as the current WhatsNew metadata for the App class.

Things like fastlane supply and Google Play support a "What's New" entry
per-APK, but fdroidclient does not current use anything but the current
version of this data.  Right now, it seems we probably only want to have
the latest WhatsNew in the index to save space.

In theory, we could make the WhatsNew data structure follow the structure
of fastlane/Play, but that would quite a bit of complexity for something
that might never be used.

fdroidclient#910