Refactors the way music folders are handled

- Second part of the fix for tsquillario/Jamstash#212 - Fixes getIndexes
- Moved loading / saving / deleting the selected music folder to persistence-service.
- Moved the request to get all the music folders to subsonic-service.
- Moved SelectedMusicFolder to subsonic-controller's scope from the rootScope
- Fixes a bug where we would send a '-1' music folder id to Subsonic, causing an SQL error when choosing "All Folders" and refreshing the Artists list.

app/common/main-controller.js

@@ -10,7 +10,6 @@ angular.module('JamStash')
     $rootScope.Genres = [];
     $rootScope.Messages = [];
 
-    $rootScope.SelectedMusicFolder = "";
     $rootScope.unity = null;
     $scope.Page = Page;
     $rootScope.loggedIn = function () {