BPM metadata enhancement

Related to navidrome#1036.

Adds BPM to the stored metadata about MediaFiles.

Displays BPM in the following locations:
- Listing songs in the song list (desktop, sortable)
- Listing songs in playlists (desktop, sortable)
- Listing songs in albums (desktop)
- Expanding song details

When listing, shows a blank field if no BPM is present. When showing song details, shows a question mark.

Updates test MP3 file to have BPM tag. Updated test to ensure tag is read correctly.

Updated localization files. Most languages just use "BPM" as discovered during research on Wikipedia. However, a couple use some different nomenclature. Spanish uses PPM and Japanese uses M.M.

db/migration/20210430212322_add_bpm_metadata.go

@@ -0,0 +1,31 @@
+package migrations
+
+import (
+	"database/sql"
+
+	"github.com/pressly/goose"
+)
+
+func init() {
+	goose.AddMigration(upAddBpmMetadata, downAddBpmMetadata)
+}
+
+func upAddBpmMetadata(tx *sql.Tx) error {
+	_, err := tx.Exec(`
+alter table media_file
+add bpm integer;
+`)
+	if err != nil {
+		return err
+	}
+	notice(tx, "A full rescan needs to be performed to import more tags")
+	return forceFullRescan(tx)
+}
+
+func downAddBpmMetadata(tx *sql.Tx) error {
+	_, err := tx.Exec(`
+alter table media_file
+	drop bpm;
+`)
+	return err
+}

model/mediafile.go

@@ -39,6 +39,7 @@ type MediaFile struct {
 	Compilation          bool      `json:"compilation"`
 	Comment              string    `json:"comment"`
 	Lyrics               string    `json:"lyrics"`
+	Bpm                  int       `json:"bpm"`
 	CatalogNum           string    `json:"catalogNum"`
 	MbzTrackID           string    `json:"mbzTrackId"         orm:"column(mbz_track_id)"`
 	MbzAlbumID           string    `json:"mbzAlbumId"         orm:"column(mbz_album_id)"`