* add disc field to songs table via migration
* add disc field to song model
* collect disc information in file sync process
* sort song list by disc and track number in album context
* check the disc field in file test
* remove nullable flag from disc field in add_disc_into_songs migration
* fix php code style issues
* fix js code style
* fix sort usage in song-list
* sort songs by inserting into queue by clicking the album/artist item play button (album_id, disc, track)
* add disc to sync tags in media model