Allow ExternalLink icons to be styled (#1503)

* Allow ArtistExternalLink icons to be styled

* Allow AlbumExternalLink icons to be styled

* Standardize external links' classes to kebab-case

Co-authored-by: Deluan <deluan@navidrome.org>

ui/src/artist/ArtistExternalLink.js

@@ -36,9 +36,17 @@ const ArtistExternalLinks = ({ artistInfo, record }) => {
     linkButtons.push(<span key={`link-${record.id}-${id}`}>{link}</span>)
   }
 
-  addLink(links[0], 'message.openIn.lastfm', <ImLastfm2 />)
+  addLink(
+    links[0],
+    'message.openIn.lastfm',
+    <ImLastfm2 className="lastfm-icon" />
+  )
   artistInfo?.musicBrainzId &&
-    addLink(links[1], 'message.openIn.musicbrainz', <MusicBrainz />)
+    addLink(
+      links[1],
+      'message.openIn.musicbrainz',
+      <MusicBrainz className="musicbrainz-icon" />
+    )
 
   return <div>{intersperse(linkButtons, ' ')}</div>
 }