Parse date attributes in appdata
Parse date attributes in appdata

The spec allowed either timestamp or date to
be specified for a release, so we should support
both.

Closes: #2614

Closes: #2619
Approved by: alexlarsson

flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak