screenshots for gnome-software and kde discover

Software package managers like gnome-software and Discover (kde) support have support screenshots and we want our Jami screenshots there to help users to have a good preview for our app.

Definition of done:

1. Create our official screenshots
2. Git repository for media (done)
3. Screenshtos in git repository (done)
4. Configure Jenkins to deploy media repository in https://dl.jami.net/media-resources
5. Add the metadata with the screenshots urls in Jami-gnome
   • https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html
   • https://git.jami.net/savoirfairelinux/ring-client-gnome/blob/master/jami-gnome.appdata.xml