Load cover art from file directory

This commit adds support for loading cover art from media file
directories, according to configured filename priorities (of which an
additional, special choice of `embedded` is given).

Cover art paths are resolved during scanning and stored in the database
as part of the `album.cover_art_path` column; if embedded cover art is
matched, this will default to the path of the media file itself, and if
no cover art is matched at all.

Similarly, the `album.cover_art_id` column will default to a reference
to `media_file.id` if embedded cover art is wanted, but if an external
cover art file is matched, this will instead be set to a reference to
the `album.id` value itself, prefixed with the `al-` constant.

Stored cover art paths are once again resolved and matched against
configuration when covers are requested; that is, any change in
configuration between scanning and requesting cover art may not return
correct data until a re-scan is complete.

Tests will be added in future commits.

conf/configuration.go

@@ -33,6 +33,8 @@ type nd struct {
 	ImageCacheSize          string `default:"100MB"` // in MB
 	ProbeCommand            string `default:"ffmpeg %s -f ffmetadata"`
 
+	CoverArtPriority string `default:"embedded, cover.*, folder.*, front.*"`
+
 	// DevFlags. These are used to enable/disable debugging and incomplete features
 	DevLogSourceLine           bool   `default:"false"`
 	DevAutoCreateAdminPassword string `default:""`