renamed model hooks
added content to Song model, refactored db schema, refacoted current_songs filter to current_release filter, refactored releases both admin and non-admin, refactored songs both admin and non-admin
refactored a lot of things
removed useless db schema and code
syntax tuned
renamed update statics method of every model to "update_props", modified related code in controllers
added missing package need to be required
added contents to Artist model, Artist model hooks, Link model and instance methods, refactored both admin and non-admin artists controller, added missing db schema, modified field name in _fields.html partial of artists for matching schema