- Added missing_album_cover.jpg placeholder image.
- Modified the graphics helper to use it. Calling graphics::generate will copy it.
- Modified item::remove_album_cover and gallery_event::item_created to run graphics::generate.
- Added unit test to Graphics_Helper_Test.
- 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.
- Made missing_photo match the image format (jpg, png, etc.).
- Swapped missing_photo.png for missing_photo.jpg since it's likely to require less conversion to match.
- Improved error messages to user when things go wrong.
- Ensured that missing image placeholders are always copied when there's an error.
- Ensured we don't mistake no file output for a correct file output (delete target before attempt).
- Restructured graphics::generate a bit to work better with above changes.
- Added unit tests for graphics::generate.
