added thumb_extension and resize_extension to items

- added thumb_extension and resize_extension to items
- added gallery variables to set default thumb_extension and resize_extension assignment
- consolidated methods of album cover thumb generation to one
- made resize_dirty consistently 0 for movies and albums
- edited item model and file_proxy controller
- edited item, data_rest, item_rest, legal_file, graphics, gallery_task, gallery_installer, and gallery_event helpers

@todo: run packager to get new install.sql
@todo: create unit tests
@todo: test