Validation, settings UI cleanup and different settings models, oh my

New: Download client UI matches other settings
Fixed: Prevent drone factory folder from being set to invalid paths/root path for series
Fixed: Switching pages in settings will not hide changes
Fixed: Test download clients
Fixed: Settings are validated before saving

src/UI/Settings/DownloadClient/DownloadClientSettingsModel.js

@@ -0,0 +1,11 @@
+'use strict';
+define(
+    [
+        'Settings/SettingsModelBase'
+    ], function (SettingsModelBase) {
+        return SettingsModelBase.extend({
+            url           : window.NzbDrone.ApiRoot + '/config/downloadclient',
+            successMessage: 'Download client settings saved',
+            errorMessage  : 'Failed to save download client settings'
+        });
+    });