VGMdb importer should lookup the VGMdb URL before offering an import button

[yindesu]

For example, https://community.metabrainz.org/t/soundcloud-import-userscript/618296/1 replaces its "import" button with an "open" button if the SoundCloud URL is found in MusicBrainz. The VGMdb importer should ideally do the same, as many users of this script are unable to find preexisting MusicBrainz releases, some of which were already linked to the VGMdb page.