post: Panic app uninstall and repo reset

This describes the new features in fdroidclient!827 (merged). There is a related post here https://guardianproject.gitlab.io/info/2019/06/04/panickit-1.0-built-in-panic-button-and-full-app-wipes/

@grote @cde @jefflandale @nickcalyx this is also mentions CalyxOS's work here.