ReplayGain support + audio normalization (web player) (#1988)

* ReplayGain support

- extract ReplayGain tags from files, expose via native api
- use metadata to normalize audio in web player

* make pre-push happy

* remove unnecessary prints

* remove another unnecessary print

* add tooltips, see metadata

* address comments, use settings instead

* remove console.log

* use better language for gain modes

scanner/mapping.go

@@ -75,6 +75,13 @@ func (s mediaFileMapper) toMediaFile(md metadata.Tags) model.MediaFile {
 	mf.CreatedAt = time.Now()
 	mf.UpdatedAt = md.ModificationTime()
 
+	if conf.Server.EnableReplayGain {
+		mf.RGAlbumGain = md.RGAlbumGain()
+		mf.RGAlbumPeak = md.RGAlbumPeak()
+		mf.RGTrackGain = md.RGTrackGain()
+		mf.RGTrackPeak = md.RGTrackPeak()
+	}
+
 	return *mf
 }