Expose 'Last Updated' date to gnome-software for snaps

[Amr Ibrahim]

Please expose the 'Last Updated' date to gnome-software for snaps. That date is shown on the online store https://snapcraft.io/store for snaps.

Rationale:
Some snaps have not been updated since several months (example https://snapcraft.io/jabref, last updated on Aug 18 2017), so that date gives an indication to users whether the snap is still maintained by its uploader or not.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gnome-software 3.28.1-0ubuntu4.18.04.4
ProcVersionSignature: Ubuntu 4.15.0-43.46-generic 4.15.18
Uname: Linux 4.15.0-43-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Thu Jan 24 20:29:01 2019
InstallationDate: Installed on 2018-11-29 (56 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
InstalledPlugins:
 gnome-software-plugin-flatpak N/A
 gnome-software-plugin-limba N/A
 gnome-software-plugin-snap 3.28.1-0ubuntu4.18.04.4
SourcePackage: gnome-software
UpgradeStatus: No upgrade log present (probably fresh install)

[Robert Ancell (robert-ancell)]

GNOME Software doesn't have a field to show this, so this needs some thought as to how it fits into the upstream design.

[Amr Ibrahim (amribrahim1987)]

The Flathub store for flatpak apps also have that date, so it would make sense to show the 'Last Upload Date' for both, snaps and flatpaks.

[Robert Ancell (robert-ancell)]

Contact URL was accepted into the AppStream spec:
https://github.com/ximion/appstream/commit/556f01190df1a9158b2532e5ee23a9f2f24ae1f6

[Robert Ancell (robert-ancell)]

Oops, that last comment was meant for another bug.