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

db/migration/20230117155559_add_replaygain_metadata.go

@@ -0,0 +1,34 @@
+package migrations
+
+import (
+	"database/sql"
+
+	"github.com/pressly/goose"
+)
+
+func init() {
+	goose.AddMigration(upAddReplaygainMetadata, downAddReplaygainMetadata)
+}
+
+func upAddReplaygainMetadata(tx *sql.Tx) error {
+	_, err := tx.Exec(`
+alter table media_file add 
+	rg_album_gain real;
+alter table media_file add 
+	rg_album_peak real;
+alter table media_file add 
+	rg_track_gain real;
+alter table media_file add 
+	rg_track_peak real;
+`)
+	if err != nil {
+		return err
+	}
+
+	notice(tx, "A full rescan needs to be performed to import more tags")
+	return forceFullRescan(tx)
+}
+
+func downAddReplaygainMetadata(tx *sql.Tx) error {
+	return nil
+}