CMS-4618 Media Types - Introduce input type FileUploader

- refactored UploadItem<Model>
- refactored api.ui.uploader.Uploader to use new UploadItem
- added onUploadFailed event to Uploader
- deleted api.ui.uploader.FileUploader
- updated api.content.MediaUploader to show links to uploaded items (link is not working yet)
- updated ImageSelector input type to handle upload error