#1968 - Improve album cover generation/removal/etc.

- Added stanza to Item_Model::save that handles when cover id is null.
- Added logic to graphics::generate to copy/convert album cover thumbs from their item thumbs to ensure they're always jpg, and eliminate the possibility that we copy/convert a dirty thumb.
- Redirected other places in code where we want to do one of the above two things to use these two functions instead (gallery_event::item_updated_data_file, item::make_album_cover, item::remove_album_cover).
- Improved validation in Item_Model so only albums can have covers and all covers must be non-albums.
- Added unit tests to Graphics_Helper_Test.