Hints for gnome-sushi in universe

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• asv-unknown-tag
  org.gnome.NautilusPreviewer.appdata.xml:71 - updatecontact
  Found invalid tag. Non-standard tags should be prefixed with `x-`. AppStream also provides the <custom/> tag to add arbitrary custom data to metainfo files. This tag is read by AppStream libraries and may be useful instead of defining new custom toplevel or `x-`-prefixed tags if you just want to add custom data to a metainfo file.