asgen-config: add MediaBaseUrl for remote media to be downloadable
Otherwise remote media like remote icons and screenshots are only reference the original origins, having a much worse data quality.
https://github.com/ximion/appstream-generator/blob/master/docs/asgen-config.md#description-of-fields
In principle this should have been enough for screenshots to show in GNOME Software. Unfortunately the expected behavior was not documented and a patch is needed in GNOME Software. I will do that in the following days/weeks. But at least once the fix lands in GNOME Software, the appstream data will be of enough quality to display screenshots and remote icons :)